we cannot think that Tybalt's blackmail, which in reality is not blackmail but advice, has no value.
so either the student environments, such as classrooms and libraries, constitute a free zone (in the end it is not that Jill can "escape" when she meets MC), or Jill has not actually accepted Tybalt's advice yet, waiting to talk about it with Mc ( which would make the whole question less "silly")
however apart from that, entering a dressing room would already be a risky gesture normally, in Jill and MC's situation it would be a real gamble. Tybalt doesn't need to see them, but he just needs to know, and since the requests he made to Jill were just to spend time together, it's not out of this world that he accompanied her shopping, naturally waiting outside the dressing rooms.