Duo says that "a few students" is translated as "qualche studente" (sing.)? Is this correct?
Yes, "qualche" always needs a singular, even though it always means a plural.
Now that's confusing!
In German you will encounter the same problem with mancher (sing) <> manche (pl) Singular means "a lot", plural means "some"
Would it also be OK to say, "AT that school"?
Don't see why not.