Given you think that schools are should be subject to the law in these terms, you are not an absolutist on them making the decision. I think free speech, and its limitations need to be the place of the wider society, not specific interests.
I suspect that we both agree that a school will have some devolved power to impose more restrictions than the wider society but what is devolved is surely determined by the society not the school?
Sure - society, via policy makers and the law may determine that certain powers are devolved to a school. But that isn't the question - the question is who actually exercises those devolved powers and makes those discretionary decisions. You are still not answering that question - 'society' has no mechanism to make specific discretionary decisions on what rules a school does or does not set. How would that work?
And where there is authority (in this case to make those discretionary decisions) there should also be accountability for those decisions. 'Society' (whatever that means) cannot be held accountable for those kind of decisions - but individual decision makers, e.g. the leadership and governance within a school can.
So rather than vague hand waving about 'society' why not answer the question I asked - here are two discretionary decisions associated with schools.
1. Should a school allow muslim prayers during the school day and on school premises - yes/no/sometime etc
2. Should the school have a strict uniform policy for its pupils.
Who should make those decisions - if not the school, then who? (Noting that there must be a sensible mechanism for the decision maker to make the decision and that the decision maker should be able to be held accountable for the decision).