If you believe that only a secularist would find this inappropiate, then it is you who has the narrow view. High School graduations are not the place for a tent revival or for any type of attempt at religious conversion. A prayer or a part of the speech being used to explain how religion played a factor in her success would have been OK. But, I think sometimes people pull stunts like this to pick a fight.
Actually, graduation ceremonies have nothing whatsoever to do with completing the requirements to be certified as a graduate.
They are "ceremonies" ~ nothing more, and ceremonial type stuff is always in order at such events.