Our daughter graduated from Groton a few years ago. The dimensions of the Groton education extend well beyond academics, which are very rigorous, to include leadership, team work, community service, and faith. Groton is peerless; while perhaps equaled in some dimensions, it is not surpassed.
Groton School is the most phenomenal school in the world. It is exemplary in producing well-rounded students with rich backgrounds in academics, leadership, service, athletics, and the arts. At the very least, it is better than St. Mark's.
Groton is a boarding/preparatory school. I currently am a student there, and I enjoy it. The hardest part for me is being away from my family, but your friends will help you fill that void. The teachers are always available, and there are a lot of different courses to choose from.