Are invocations in school events legal?

A month or so ago, I had an SGA conference in Austin, TX. It was a pretty big conference with people from all sorts of nationalities, races, and religions. My question is that before they started each general meeting, they had a person come up and do an invocation, is that legal?. I have never been quite religious and I did not bow my head, and they were praying because the guy who did it said "In your name we pray." It kinda bothered me and a few others through out the week long conference, though we didn't complain.