My point was that if you have a kid come into your classroom with their shirt untucked, you tell them to tuck it in and they say ‘no’, then what do you think will happen next? They’ll come in, sit down and work beautifully, complying with every other instruction without hesitation?
A kid who says ‘no, I’m not going to’ to a teacher issuing a reasonable instruction is starting something. Some schools would rather that the teacher spends ten minutes trying to get the shirt tucked in while the rest of the class twiddles their thumbs, or tactically ignores the refusal to follow instructions so the kid then continues to flout rules to the point where the lesson is so destroyed that the behaviour system finally kicks in.
Taking that kid out of that environment may or may not improve their behaviour, but it will sure as hell improve the learning environment for the rest of the class.
And if you’re going to go ‘school uniforms are stupid, I don’t see why kids should tuck their shirts in anyway’ then replace ‘tuck your shirt in’ with any other reasonable request to follow a school rule that a teacher might issue.