The pronouns in the sentence are: most, that, and who.
None of these have a vague pronoun reference. The reference for each of these pronouns are:
- most - an indefinite pronoun that takes the place of a noun for an exact number of students.
- that - a demonstrative pronoun which refers to 'classes' that are asynchronous.
- who - a relative pronoun that introduces a relative clause that gives information about its antecedent 'students'.
