At Hyde there is great pressure put on kids to return for the "post-graduate" year. Actually, it has been decades since it was called that. Hyde came up with a very slick way of restructuring the grades to ensure a higher yield of returning students.
What would ordinarily be called your Senior year, is now called "Senior Prep." If your character development has been outstanding, you might be lucky enough to become a full-fledged Senior partway through the year. More often than not, you will not, and at the end of that year you will be given the choice of graduating with a "certificate" (somewhat analogous to a GED), or returning for an extra year in the hopes that you will make it to Senior Leadership, and possibly be able to earn a Hyde diploma by year's end.
I believe Junior year is now also stratified like that, although less distinctly. There is a term they use for those ready to become Senior Preps but it escapes me at the moment.
As if that were not bad enough, there have been cases of students held back a year before even getting to that point, by virtue of their grades being artificially depressed via the character component. One gets graded on both academics and character development, and of the two, the latter is more important. Moreover, even though these two grades are supposed to be judged separately, they aren't really. If you're a kid who is deemed to totally suck in the character department, guess what, you can kiss an "A" in anything goodbye, even if you are the world's next Einstein and had to teach the class half the time (because the teacher didn't exactly know the material).
It is certainly feasible that a kid could do six years there going through the Freshman-Sophomore-Junior-Senior years, although it is more likely to be five. I did know a 20 yo student in my Senior Prep class, however, who was still at that level by virtue of Hyde's system, and I don't think he ever made it to the diploma stage.