Individual freedom of expression has a place in a university. Institutional freedom of expression (in general) does not. When a nun or priest in uniform comes to a university, they are representatives of the church. To be there, they have to ask permissions (and if the university is any good, it would deny this permission).

A student can dress up as a nun, or priest, or nazi, or wear no clothes or whatever, and I will support his right. But the church institution, or political parties, or the army have no place there. If a nun does non-church business, she should wear civilian clothing.