Loosing 70 percent of your class in any academic setting is actually pretty common. Of my freshman class in university only 25 percent of us graduated. Public schools, high schools that is, tend to have their own drop out rates and student transfers. What those numbers are I have no idea. Given that they are public institutions I wouldn't compare them to a private duckfarm like Hyde.
Hyde survives not by the number of students that graduates, but by the number of students that walk in the door as new students every year.
So long as hyde can keep its overall population over a certain level the number of graduating students is of no consquence as I'm sure Hyde has become quite adept at manipulating the facts to make some sort of plausible explanation avaliable to those few parents who even bother to ask.
More than likely the majority of our current parenting herd would be content with general statements like, "Well 90 percent of our graduates go onto college!"
"coughs... don't mind the fact that only 30 percent actually survive this shithole."