Here's the full reference in the California legal code. The main section:
- (a) A school district operating one or more high schools, a charter school, or a private secondary school shall not make or enforce a rule subjecting a high school pupil to disciplinary sanctions solely on the basis of conduct that is speech or other communication that, when engaged in outside of the campus, is protected from governmental restriction by the First Amendment to the United States Constitution or Section 2 of Article I of the California Constitution.
There's a clause related to religious schools:
(c) This section does not apply to a private secondary school that is controlled by a religious organization, to the extent that the application of this section would not be consistent with the religious tenets of the organization.
In my layperson research most references I found blanket discount the entire section because of clause '(c)', saying the entire section (known as Leonard Law) doesn't apply to religious schools. To me that doesn't seem correct.
Yes, at a catholic school students couldn't hold a "pro-abortion" rally on campus because abortion is inconsistent with a tenent of the church.
On the other hand, a rally in support of a teacher or student that was just dismissed (or expelled)... there's nothing there that goes against the tenents of the religion.