You equate Christian prosyletizing with a "How I Lost my Virginity" speech?! That's just assinine.
The point is that it is her speech. Some people in the crowd may be offended by a speech defending capitalism after all they've been taught in the socialist school system. At what point do we say "enough" and allow people to express themselves and their ideas within the bounds of decent discourse? Again, how long until the school simply gives a valedictorian the speech they will give to ensure it is watered down enough for the masses? Whose speech is this anyway?
Besides, the valedictorian earns the right to speak on behalf of her fellow students because of her achievement. The schools think others can learn from them. If their Christian faith is what drives and motivates them, how can that be off limits? In the good ol' U S of A, it sure shouldn't be!
People have very different ideas about what constitutes "decent discourse", and the law shouldn't distinguish between those. Hence Fred Phelps and his sorry excuse for a church continue to run around the country delivering what they believe is an urgently important religious message. What should a school do if a student with that sort of religious beliefs earns a valedictorian spot? Even with specific recognized obscene words barred, and replaced with respectable synonyms, most people would much rather be subjected to the "How I lost my virginity speech. And what about white supremacists, who certainly have a right to express their political views, and who often practice their beliefs within the context of an organized religious group?
I'm not suggesting that schools should be allowed to apply a different standard to religious speech by valedictorians, than to any other type of speech. But I think it's also acceptable for a public school to apply a standard across the board, limiting valedictorian speeches to material which wouldn't offend any significant number of reasonable people in the community. And while I don't think it's reasonable for anyone to be offended by a student describing his/her own religious faith and the role s/he believes it played in his/her academic success, I do think it's reasonable for anyone who doesn't share the student's specific religious beliefs to be offended by a speech which amounts to aggressive proselytizing. This is why I think we need to know what this student was startig/planning to say, before passing judgement. An auditorium holding many Catholic, Jewish, atheist, agnostic, and other non-evangelical Christian parents who are their to witness their own children's graduation, shouldn't be given a choice between listening to speech which the speaker and the school officials both know will offend them, and getting up and leaving their own child's graduation. Neither should an auditorium full of religious parents be forced to sit through a militant atheist diatribe, in which the speaker admonished them and graduating classmates that they can't aspire to high academic achievement unless they give up their stupid superstitions.
The refusal of courts to allow public schools to set reasonable limits on students' speech, behavior, and dress, has been a huge contributor to descent of public schools into an anti-civilizing force. It's only in very recent years that courts have begun to uphold dress codes. For a long time, if a girl wanted to go to school in a teensy midriff-baring halter top and teensy low-rise "hot pants", the school wasn't allowed to interfere with her right to "free expression". Finally, courts started coming around to the idea that reasonable limits on student attire were appropriate and permissible, and they should come around to that idea about ALL kinds of speech as well.