I'd love for you to explain what exactly you would have liked the teachers to have done differently.
They were loading the buses for the kids to go on a trip. Literally ready to go.
Your son and his friend do something - you won't tell us what - that is considered sufficiently naughty to make the teachers decide in the moment that they couldn't go on the trip.
But they've got however many other kids right there who need to be loaded on the buses, counted on, names ticked off registers, medicine checked, seatbelts checked, etc. That would easily take up all the attention of the staff there in the moment.
So what can they do with your son and his friend?
Can they send them off, unaccompanied, to another part of the school to wait, not knowing if a member of staff will be in that room/space to supervise them?
No. That wouldn't be safe.
Can one of the teachers who is present and accompanying the trip be spared to take the two boys inside, and wander around for a while trying to find an appropriate member of staff to supervise them, while the other teacher/s loads the buses?
The teachers in the moment clearly decided that no, they couldn't spare that teacher, for the safety of the other students.
So, the most expedient solution was to have the two boys sit on the bench where they could be safely supervised, while the coaches were loaded with the other students, registers taken, seatbelts checked, etc - and then they would deal with the problem.
Totally understandable, totally acceptable.
Your son created an issue by misbehaving. The teachers then had to deal with that issue in the moment, in a busy and stressful situation, as best they could, considering the needs of all the students for whom they were responsible.
It's unfortunate that your son started crying, his friends saw him, and he was embarrassed.
But that's the price he chose to pay for misbehaving. No one made him misbehave.
He wasn't deliberately humiliated. He wasn't forced to sit on the bench to make a point.
He was sat in the only place that was convenient and safe for him to be for a few minutes while the teachers sorted out the situation.
That is it. Not worthy of any kind of complaint. The apology in this situation should really be coming from your son, for causing inconvenience and stress to his teachers, for goodness' sake!