"Your anecdotal experience does not negate the fact that school administrators regularly limit what students may say at school functions."
Are they unlimited in this ability? Can they absolutely control what a child may say? If they are limited, what are the limits?
I have no idea what the legal limits are, but courts have regularly sided with administrators who have prevented the wearing of certain colors, of t-shirts with certain messages on them, and of the handing out of leaflets or certain kinds of information.
If this valedictorian was not comfortable giving her talk without the references to Jesus in it, she simply should have declined to speak at all, rather than say she would give the edited speech, then not give it.