In general, a creative work like a photograph is owned by the creator of the work, even if they are a student journalist. However, the school might have some rights to use a student journalist's photos, especially if they signed an agreement. 

The Free Speech Clause of the First Amendment protects your expression. But, the First Amendment also has an Establishment Clause, which bars any government actor, like your public school, from promoting an establishment of religion. In general, schools have wide authority to shut down speech solely because it is religious. Whether they can shut down speech under the Establishment Clause or if it is protected under the Free Speech Clause depends on two questions: (1) whether the students hearing the message are a “captive audience,” and (2) whether the student’s statement constitutes religious speech.