This is true of all educational settings -- except that the language subset then becomes an unenforced (and hence very leaky) abstraction. Students inadvertently step outside the subset and get either bizarre errors or, worse, their program runs and produces strange errors.

DrRacket pioneered the notion of "language levels" that grow with the student's needs. Pyret will end up taking some variant of this route. It's just too early to design those levels yet.