Rumor has it that a very enthusiastic student and a faculty/staff potential coach are exploring a Varsity Ultimate team for The HIll this coming fall. I know Lawrenceville has a team, what other schools do? Other Mid Atlantics? It would be boring for them to just compete against L’ville all the time! 
Choate has a lively ultimate team. Andover does as well.
Most schools post their athletic competitions on the website. You should be able to see what schools they tend to play. Some will play local day schools and public schools as well as other boarding schools in ultimate.
Exeter has a club team. Leave it to my son to get a concussion playing his senior year.
The captain of the Choate team (pretty sure he was the captain) graduated at the very tippy top of the class of '15. Obviously, playing Frisbee improves your class rank. 
DS1 loved ultimate his lower year at Andover. He did not play upper or senior year though, the full week end long matches were tough on the academic schedule. He found ultimate much harder to balance than varsity soccer because of the full week end events. He is picking it back up an Penn though and even called home to check in on how mom and I feel about him pledging the “ultimate” fraternity.