I sailed in HS and College in the US, as well as coached a HS team. I traveled to events where the host school had outlier boats (not FJ or 420). My experience has been that the outcome of the event is not vastly different than it should be. Meaning the good schools that normally win still win, the not so good schools trail the fleet. Conversely when the school that has outlier boats travels and races on FJ's or 420's their result isn't that different than when they sail at home. Imagine that.
Two reasons to stick FJ's or 420's that have been covered are cost and parts availability, vast majority of scholastic programs are budget limited. One big reason I think is team racing. If you have a fleet of 49ers they're going to get busted up, fast if you have kids team racing them.
FJ's and 420's are not exciting at face value. But I have yet to encounter anyone that was genuinely interested in sailing that didn't step up into a faster boat when the time in life came. Truth be told during my coaching stint some 15 years after college I would occasionally hop in the 420 on breezy days during practice, I had a hoot doing so.