I think that the Boggart just sort of knows the biggest fear of its victim because I think it does this to scare of enemies and these might not always be thinking about their biggest fear when seeing a Boggart, for example they could just stumble upon a boggart without knowing its a boggart and then they would really be in for a scare.
Do you think that the Boggart assumed the form the students were expecting because at that moment, they were thinking about it and were convinced it was their biggest fear? If they had run into a Boggart without knowing it, do you think any of the forms it assumed would be different to what we saw in the lesson?
I think this is due to the fact that the other students are not constantly reminded of Voldemort and most don't have traumatizing memories where he kills their parents (unlike Harry who does). They probably fear Voldemort just as much, but in their knowledge, he is gone and dead (well in that class in third year he was... supposedly)
Say, Lupin said to Harry the reason why he wouldn't let him face it was because he thought it would turn into Voldemort and that would cause panic, indicating that people would be more afraid of Voldemort than their actual Boggarts, so one could ask how come the Boggart didn't turn into Voldemort at one point anyway?