When does an argument against vegetarianism become harassment akin to racial harassment — based on someone's belonging to a particular group?

Eugene Volokh calls attention to the case of a 6th-grader who was punished — 5 lunch-hour detentions — for telling a fellow student "it’s not good to not eat meat" and that "he should eat meat because he’d be smarter and have bigger brains" and "vegetarians are idiots."

An administrative judge wrote that the boy's statements "were reasonably perceived as being motivated by a distinguishing characteristic between the two boys, namely vegetarianism, which substantially interfered with the rights of K.S. and had the effect of insulting or demeaning him."

Volokh observes that the judge did not stress the empty insult "vegetarians are idiots," but "treated this statement as on par with polite factual and normative claims (whether accurate or not), such as 'it’s not good to not eat meat' and '[you] should eat meat because [you]’d be smarter and have bigger brains.'"

So would the boy have been punished if he'd limited himself to a substantive argument against avoiding meat? Isn't it significant that vegetarianism can be part of a person's religion or religion-like in its importance to a person? Are children allowed to proselytize in the lunch room? Does it depend on whether they can refrain from insults or do we just not want them arguing about things that go to the deep core of human identity? The question is: How big is this notion of harassment and how small — by contrast — are the free speech rights of school children?
These questions made me remember something Justice Thomas wrote back in 2007 (in the "Bong Hits 4 Jesus" case):

As originally understood, the Constitution does not afford students a right to free speech in public schools.... If parents do not like the rules imposed by those schools, they can seek redress in school boards or legislatures; they can send their children to private schools or home school them; or they can simply move. Whatever rules apply to student speech in public schools, those rules can be challenged by parents in the political process.

In place of that democratic regime, [Tinker v. Des Moines Independent Community School Dist., 393 U. S. 503 (1969)] substituted judicial oversight of the day-to-day affairs of public schools.... Tinker has undermined the traditional authority of teachers to maintain order in public schools. “Once a society that generally respected the authority of teachers, deferred to their judgment, and trusted them to act in the best interest of school children, we now accept defiance, disrespect, and disorder as daily occurrences in many of our public schools.”...

That case was about a silly statement about drugs (with some stray religion). Other cases are about bullying and what counts as bullying. A school might choose an expansive definition of bullying and demand that students be very nice to each other and to respect each other's values and beliefs. As long as there isn't viewpoint discrimination — give detention to the vegetarian who tells the meat eater he's running his life the wrong way — then I don't see what's wrong with setting the rules of deportment.