In this case, 'who' and 'that' are mostly equivalent. If you're talking about a specific person, sometimes 'who' is the only choice, but here it probably doesn'

In this case, 'who' and 'that' are mostly equivalent. If you're talking about a specific person, sometimes 'who' is the only choice, but here it probably doesn't matter. 'Which' wouldn't be used here because 'are studying language pedagogy' is restricting/filtering 'students', and so it's essential to the meaning of the sentence. 'Which' is used for clauses that add non-essential information. My shirt that is red is upstairs. My shirt, which is red, is upstairs. The first sentence is contrasting the subject with other shirts (..., but my blue shirt is not). In the second sentence, there's only one shirt being discussed - it happens to be red, but even if that information was left out, it wouldn't change what shirt you were discussing.