Let's put a different spin on this.
Let's suppose that some member of the graduating class is an atheist. And what do Christians think is going to happen to an atheist in a church? They probably think they'll get struck by lightning.
So, tell me, do you think that is Constitutional? Demanding that some members of the graduating class should risk being struck by lightning in order to attend graduation?
Okay, ignore for a minute whether certain religious conservatives might think they deserve it, I don't see how you can suggest that it was Constitutional.
Christians like to complain about atheists and will use any excuse to do so. They whine about 'why should atheists care about going into a church is they don't believe in it?' But the point is to make this argument about what Christians believe, and not what atheists believe. Christians essentially are saying that atheists will desecreate a church merely by stepping inside (thus the lightning), so why would they think it's a good idea to have a secular ceremony there. If an atheist decides they aren't going to be intimidated by the overtly religious surroundings, the Christians are the ones who should be offended.
What they really want is for us not to participate in anything at all. They want us to go away. But it's really amazing to me how they can rationalize any position they want. They need to learn proof-writing.