Many people who are not Catholic and even atheist send their children to Catholic school - the religious content in most Catholic schools is minimal and some parents may agree with the more disciplinarian approach to education. I know three or four Episcopilians (sp?) that went to Catholic school.
And as far as government-sponsored forums, like public airwaves and, yes, high school commencements, the government is free to apply whatever arbitrary standards it chooses. Women are free to walk around in public without their tops on, but let a nipple make a two-second appearance on the airwaves, and that's a million-dollar fine from the FCC. Similarly, at a commencement, do you really think that a commencement speech that consisted of many repetitions of the word 'fuck' would be allowed? It's not exactly an example of the highest level of discourse (then again, neither is Celine Dion), but any difference between the profanity-laden speech and the song is essentially arbitrary.
That being said, the government does have the responsibility not to use these admittedly arbitrary standards not to silence serious viewpoints - it should not be in the business of taking programs critical of the government off the air. I will admit tacitly that Christian viewpoints in general are being mostly excluded from schools. I do not think that is a problem - free speech laws were designed to protect minority viewpoints and Christianity is in no way a minority viewpoint.
However, you have every right in the world to lobby against encroaching secularism in schools. But to take the example of a song at a commencement as an example of this secularism is silly. Commencements are as sanitized as the school board can get them. Christians delegitimize their own arguments by using these types of things - like no Christmas songs at holiday pageants. Who really cares if you don't hear 'Silent Night' for the umpteenth million time? For the same reason that not many people really care if the FCC is encroaching on Howard Stern's 'free speech' rights, not many people really care if Celine Dion (who gets too much airplay as it is) or Silent Night is not sung in public schools. All I'm saying is - pick your battles. Wait until schools do something like cancel extracurricular prayer circles. Until then, you're just the boy who cried 'wolf.'