"Seeing as this event was organized by the school, an arm of the government, the school has a right as a matter of policy to control the content."
Good point - then you wouldnt disagree that the school could also support her in her right to say this. Again the community, i.e. in this case the school has the right to determine the content, not you, not the ACLU, or an unelected judge.
Absolutely correct, assuming that they supported the right of anyone else to do the same with respect to their belief system. So yes, certainly the school could adopt a (stupid) policy of allowing students to evangelize during graduation speeches. However, in this case they did NOT do so -- they explicitly state that the girl's behavior violated their policy. And a school that encouraged such hijacking of their own graduation ceremonies would probably get a lot of complaints from the parents & other community members.