Coming back to this after a while, I do finally have to agree that it won't ever work. The majority of teenage characters consist of gangsters and gangsters wannabe's that show absolutely no fear of death or whatsoever. And like an admin said long ago, people come home from school to do something fun, not learn even more.
Perhaps the property could be changed into a Country Club for Los Santos? Add an indoor gym, pool, etc, make it into a nice location.
And if you really, really want a school, repurpose the County General Hospital, or the Mall close by it. To be fair, using the County General Hospital building as a school would be even neater if the Skate Park was returned, where teenagers could ride their BMX's around during breaks or after school.