“but rarely if ever do they rebuke those who make too many chumras.”
Actually, they do that all the time. Just look at the username of the poster before you.
“If you prohibit the permitted people will find out it was just some chumra and that you lied and begin to question other things you have said, Maybe they were “Chumras” too”
That is very true if you lie about the reason why you are forbidding something. For example, if you claimed that it was meforsha halacha in the Shulchan Aruch that you can’t wear these kinds of shoes.
That’s not what’s happening here. Schools are allowed to make dress codes. The school presumably has a uniform too even though there is nothing in the SA about it.
Presumably, the principal had a good reason for the rule and would be happy to explain if Johnny asks her. Maybe she even did explain and Johnny didn’t hear or understand the explanation.
In any case, we are not allowed to assume or to say that she had no reason for this rule.