In all the present tense forms, simple, continuous and perfect, who is followed by a verb in the 3rd person singular form:
Who studies English?
Who is studying English?
Who has studied English?
We use the singular verb form even when we are fairly sure that the answer will be plural (as in when we ask a group of children; "Who likes birthdays?").
Occasionally in conversation, in a form of 'echoing', who may be heard followed by a plural verb form:
A: Bill and Mary are coming to tea this evening.
B: Who are coming?
It may be heard, but that does not mean it is correct!