I am not currently a teacher but I worked in education for twenty years.
In most secondaries there is a behaviour policy. It might be called something else - sometimes it's discipline or part of the inclusion policy or whatever but all secondaries have one.
This states what sanctions teachers are supposed to give for specific behavioural problems.
For example, the behaviour policy at the last school I worked at said that if a student turned up not in uniform they had to be sent to pastoral support. Pastoral support then dealt with it, I actually don't know if they phoned home or what but the pint was teachers didn't deal with it.
Another example, the behaviour policy said that teachers were not to give out detentions without warning students first. So we all had to go to training on the behaviour policy and the example was;
Freddie, you are talking while I am talking. I am warning you now that if you do not stop you will have a detention.
(If Freddie stops - no detention)
If Freddie doesn't stop then you give a detention.
Individual teachers have some discretion over applying the behaviour policy, but if you don't apply it properly you will wind up in trouble. So for example say you set a detention without giving a warning. If the parent phones in and complains, the school policy was to cancel the detention if no warning was given.
Secondary schools employ a lot of teachers. In my last school there were over 100. In order to get some level of consistency they are told to stick to the behaviour policy and in general most of them do.
Many year 7's find this very difficult. Someone they have only just met is telling them off. They respond emotionally by feeling this person is scary.
In practice it's more like applying a customer service policy. If you bought the item over a year ago you are not getting a refund, if you turn up to school in wrong uniform you will be sent to pastoral support. Policy is policy no matter who you hassle.
It's not personal (most of the time)