Wait till the Church of Satan holds a ritual at centerfield after a game. Or a Muslim coach lays out prayer rugs and has her players face Mecca and kneel with her. Can of worms: Open.
The establishment clause prohibits government from helping or hindering religion. In this case the government (school) wanted to stop the coach from public prayer. The court ruled he was exercising his free religion/speech. Government did nothing to promote religious activities.