At first, Erin Cervelli wasn’t even considering this school. She and her husband were going on all the tours together — but not this one. “I told my husband he didn’t need to come,” she says. “I was only checking it out because it’s [relatively] close to home.”

But that tour changed everything. They’d been looking at all the “good” schools — schools with strong reputations and Spanish immersion programs. This school was neither trendy nor language-based, yet it seemed ideally suited to how their son Jack learns. Cervelli did her research, talking to friends and other parents and attending orientations and pre-school play dates. When it came to filling out the district application and ranking her favorite schools, she put San Francisco Community Alternative School first and easily nabbed a spot for her son. But she admits she was shocked when she later learned that its GreatSchools Rating — 4 out of 10 — was so low. The mediocre rating didn’t jibe with what she saw — or with what the school promises.

What’s the big deal about this rating?

Every year, 44 million people visit GreatSchools. Their goals vary, but many want to learn more about schools available in their area or where they might move. Some also want to gain more insight into their child’s school. With the exception of certain areas, including Indiana and Washington, DC, where we have more substantial data about things like student improvement rates and college readiness, in most of the country the GreatSchools Rating is based on averaging state test scores.

Here at GreatSchools, we’re constantly reminding people that test scores never tell the whole story, but they are one way for a school to measure the performance of their students on core academic subjects. What’s more, they’re often the only available way to objectively compare to what extent schools are delivering on their promise of teaching certain academic standards.

Clarity around 1 or 2 and 9 or 10

While state test scores are far from perfect, the results of very high-rated or low-rated schools do tell you something about the baseline academic performance of most of the students in that school. “When a school’s rating is a 2, you know that students at this school are scoring lower on state standardized tests on average,” explains Sam Brown Olivieri, who heads the GreatSchools data team. “Across all grades and subjects, students are scoring lower than most other students in the state.”

When you look at Flynn Elementary School (GreatSchools Rating 2), for example, their GreatSchools Rating by grade is consistently low: 1 for 2nd grade, 3 for 3rd grade, and 2 for 4th and 5th grades. Where there is variation in test scores, it reflects a wide achievement gap: Non-economically disadvantaged kids far outscore their poorer classmates (7 vs. 1) — the same score gap holds true for white students versus their Hispanic and black classmates. Does this mean you shouldn’t send your child here? Not at all. It just means you should find out what the school’s own standards of success are and how that dovetails with your expectations for your child. Flynn has both a Spanish immersion program and a general ed program so that complicates things, but the bottom line is that the test scores are consistently low.

Similarly, a high GreatSchools Rating signals another kind of consistency. “When a school’s rating is a 9 or a 10, you know that the vast majority of students, across grades and subjects, are scoring proficient or above — and better than most other students in the state – on that state’s tests,” Olivieri says. At John Yehall Chin (GreatSchools Rating 10), for example, all grade-by-grade ratings are a 10. Boys and girls perform at the same rates. And on the page that breaks out ratings by ethnic and socioeconomic subgroups, you see rows of perfect scores. “This doesn’t mean parents don’t need to visit, though,” Olivieri cautions — because you need to like how they’re teaching the students to achieve those test scores. But when you visit, you can focus on questions about safety, teaching styles, discipline practices, bullying, and… well, the list goes on and on.

The muddled middle: GreatSchools Ratings 4 – 7

When scores are really high or really low, the data tells a clearer story. It’s in the muddled middle — school ratings of 4 through 7 — where the data can suggest many different stories. “You really need to look more deeply at how students across the school are performing in different grades and in different subjects,” Olivieri says. “If a school has a steep decline year-over-year, that’s cause for alarm. Persistent principal turnover can cause challenges, too.”

Old success, new leadership

Case in point: Moscone Elementary School (GreatSchools Rating 6) in San Francisco’s Mission District. At first blush, it feels like a really strict urban Catholic school. Though it’s public, kids wear uniforms. At the early morning drop off, students and parents appear subdued. The school’s first principal, Patricia Martel, led with a singular focus: literacy — quickly and in English. She crafted simple, straightforward mission statements that the kids repeated daily and were translated into Spanish and Chinese for parents to learn as well. Kids don’t miss class, are respectful, and aim high. Unapologetically, Martel would tell parent on school tours, “If you don’t like a highly disciplined school, don’t choose this,” she recalls. Test scores reflected her vision’s success, rising to a GreatSchools Rating 9. During those years the school attracted a devoted following of local families and Martel became a model for principal training programs. But when Martel left, the school floundered a bit.

Now, seven years, two principals, one recession, and a much more impoverished community later, the school’s scores have fallen. For a high-poverty (84%) school, this shouldn’t be equated with failure, especially since the gap between some students and their economically-disadvantaged peers is narrow (7 vs. 5). And, while their GreatSchools Rating fell in 2011 (7 to 6) and 2012 (6 to 5), they reversed the trend last year (5 to 6). “This is [Principal] Valerie [Hoshino]’s fourth year,” Martel says, adding, “Change takes four to seven years to really become institutionalized. It took me a while, too.”

We’re just not that into tests?

When Erin Cervelli saw San Francisco Community Alternative School’s GreatSchools Rating 4, she was surprised but she didn’t regret her decision. Test scores “are not an issue for me,” she says. The parent reviews (4-star average out of 45 reviews) of this progressive mecca with a welcoming yard, lush garden, project-based learning, teacher “looping” (teachers stay with a class for two years so children are well-known), social-emotional curriculum, and a late start (9:15 am vs. 7:50 am at Moscone) tend to match her enthusiasm. A handful of reviews rail against academic shortcomings, such as a notable complaint in 2005 about kids needing to repeat Algebra I in high school. At orientation this summer, says Cervelli, test scores were addressed and dismissed quickly but efficiently: the project-based learning school does not teach to the test, administrators said, because it would require about two and a half months of valuable instruction time.

“When a school dismisses test scores, it’s time to ask more questions,” Olivieri says. “That’s when you need to ask pointed questions about what the school’s learning goals are — and how they measure them. Ask, ‘How do you know if your students are prepared for success?’ and ‘How do you measure progress and ensure that all kids are learning?’“ she suggests. You can also ask if the school teaches to the state standards — and if and when your school will be teaching to the new Common Core State Standards. Don’t just let it go, Olivieri says, firing off more questions to ask: “If you don’t use your state’s standardized test as a true measure, then what do you use? Do you track your students in high school and college to see how your students do? How do you hold yourselves (teacher, principal, and school) accountable for student success?”

Taking Olivieri’s advice, I asked Nora Houseman, San Francisco Community Alternative School’s principal, these questions. “We’re not anti-assessment — not at all,” she says. “We believe in assessments that are linked to what the kids are learning … but we’re very opposed to tests that aren’t.” Their project-based approach emphasizes depth over breadth, critical thinking, and presentation skills. “I think, and I think the district thinks, we’re very much aligned to the new Common Core Standards,” she says — and she’s optimistic that these new assessments will better match what they’re teaching — and that as a result the school’s test scores will go up.

Another yellow flag at San Francisco Community School is an apparent equity gap. The year before last, about a third of kids got into Lowell, one of the country’s leading high schools. But, only about one third of students aren’t from lower socioeconomic backgrounds — and the test score divide between the haves and have nots is striking: 10 vs. 3. So, does this school serve only the privileged kids? That’s not where Houseman sees the line. “There’s a big divide with our K-8 students versus our transfer students,” she says, adding that the district often recommends it to students when things don’t work out at other, more traditional, schools. “We have less time with our transfer students,” she says. “Our K-8 kids do extremely well and often get into the school of their choice.”

It’s all about your child – and getting answers

So what’s a parent to do? Look at the data to see how the school is serving children like yours. Will your son really thrive at a school where the girls are performing significantly better than the boys? Is there an equity gap? Do the test scores jump around erratically? These data points will rarely provide answers, but they can offer a starting point for figuring out the hard questions all parents need to ask.

Editor’s note: For this story, we tried to pick schools with similar levels of free and reduced lunch students — a shorthand for socioeconomic levels. But such markers — like test scores – are limited measures. There’s a clear connection in U.S. public schools between race, wealth, and student outcomes. Beyond that fact, there are other factors to consider, such as how schools serve boys versus girls, as well as kids from different backgrounds, students at different grade levels, and students with different learning needs, all of which may be crucial to consider when finding the right fit for your child.