Why is it a tough one for you? What do you not understand about free speech? Note the key word: "free." That means we're free to speak. In a public high school. Note that word, too: "public."
You try to quash my freedom to speak freely about my "particular religion," and see what happens.
You may be able to speak in a public school, but a high school kid is not free to say whatever he/she wants. If she's at a school function, the administration is free to let her speak, or not, and can ask that she not speak about certain things.
The fact is, she agreed to deliver the edited speech, then went back on her agreement.
Her parents, instead of applauding her insolence, should have encouraged her to simply decline to speak.