Tropicans will only enter High School or College if there is a (better) job available for graduates. Additionally, only High School graduates can get into College.
Thus, if you add a bunch of College level jobs (say, by building a Cathedral), you'll potentially notice a bunch of male students at the High School, who will then move to the College after they graduate.
Since jobs at College and High School require those levels of education (ie, being a College professor requires a College education), Tropicans will enroll in them to fill empty slots at the buildings themselves.
Grade School is an exception - Tropican children will enroll in Grade School without jobs available. This will increase their Intelligence, allowing them to master new jobs faster once they become adults.
Note that you're also paying your educators whether or not there are any students to train, so I often times will block off slots in the High School or College in order to keep people from working there when there aren't any jobs to train for.