The sentence is a complicated one and there are few things going
on here. First, there is metonymy, that is, reference to a thing
(here, the group of people who wake up) via the name of
something associated with that thing (here, their school). So,
the singular reflexive itself is used here, because the metonym,
Hogwarts, names an institution, which is singular (you would say
Hogwarts is (not are) lovely under the snow). Second, as Lawton
pointed out, it is the school building that was covered in snow,
not the school students, even though the school students woke
up, not the school building. That is, Hogwarts refers to
different things in Hogwarts woke and Hogwarts was covered in
snow. This is a curious but common fact about institutional
nouns. Noam Chomsky, the famous linguist, has pointed out that
you can say things like After the bank burned down, it moved
across the street, where the first clause means the bank as a
building, and the second, the bank as a business (clearly, the
burnt out building didn't