A question about school prayer

[marciosmg]

I hear about this issue a lot on american shows and movies, but I don't fully grasp it yet.

So, who can tell me what is it exactly? Or better yet, here's a question:

 

I know that a principal can't force his students to come to an auditorium and make them pray a specific prayer, but if a few students, using their free will, decide to use an empty room in school to pray, can they? If some players of the footbal team want to pray together before the match, can they?

 

Thanks in advance to whomever answers this.