[Edit: Changed Princeton to Columbia.]
No matter how great your teachers were, you forget some things and need to revise before such huge and important exam.
You're also right about the bar. Everyone I know from top-tier law schools down to those at the lowest rung schools spent an extra several months at least in week-long marathon bar study sessions.
Craziness, you would think passing the bar is something that ought to be taught in law school, not self-(re)learned prior to the test.
Theoretically, school could organize two months of sychronized revising and cramming, but I think that self revising is more effective at that age and skills level.