As amusing as I always find @Golfgr8 's poems, let me clear up a misconception that comes up from time to time: while a student may repeat a grade, it is rare that the school forces them to repeat a STEM or foreign language class they have mastered. Yes, they may be taking something called English 9 again, but I highly doubt it was the same product that they took last year at their old school.
That said, students (whether on their own or complicit with their parents) have been known to tank a placement test in order to potentially ace the course the second time around.