The only problem I would have with it is using the field in such a way that it could be perceived as school sponsored. So anything that could be seen as attempting to gain an audience for his beliefs, then the school has a right to be concerned. If he's just praying for his beliefs and offering players to join without wanting to make a big deal out of it, then I personally see that as acceptable. I really don't know one way another about the context to know which is the case here though.