@Patul Avar Assur:
I would assume Harry’s generation was particularly lower than most, as many children were killed/ people too scared of Voldemort.
It probably varied year to year, Harry’s year being the least.
But if we go with the theory that there probably under 250 students during Harry’s third year, than it doesn’t make sense that Harry didn’t notice Peter and Sirius on the map. So it must be that the map hides animagus when in animal form…