Teaching is not about 'management presentations' Clait; it is about getting students through school and hopefully at the end having managed to educate them, and get them their all important exam certificates, without which they are going nowhere fast.
I would have passed the note from the OP up to my HoD or the HoY as they would need this as evidence of consistent disruption, and they would be contacting the parent and thrashing out a way forward for both teacher and student. That is why they get paid management points.
'But I am guessing that the teacher in question doubted her own justification and therefore did not want to face the parents, which is why she didn't reply to the mother. I am also guessing that she probably tried to avoid replying to other parents, and this was creating a bigger growing problem for her as her superiors would have to start getting involved.' Key word here - guessing. You have no direct knowledge of how this was dealt with by the school; the student could have been about to be put on monitoring or report, which means that there are a set of standards that have to be met each lesson and failure to do so means an interview without coffee with either the HoY or SMT for the student.
'Surely if a teacher was ratcheting many pupils up through discipline levels over relatively minor incidents, a superior would have to start asking whether all this was really necessary.' This is precisely what discipline levels are for in assertive discipline policies, leading from a warning up to a detention with the Principal and internal exclusion. Level 3 and above are referred to the form tutor who keeps a running total (level 3 is break time detention) and level 4s are referred to the HoD and are a lunchtime detention with the HoD. HoY keep a track of the Level 4s. Level 5 and above involve HoY and SMT and perhaps internal exclusion.
Some students walked into my classroom on a permanent Level 3, and could avoid a detention by working and behaving. Failure to do so meant that they reached level 4 and 5 more quickly. If they consistently behaved and attained then they came off the permanent 3 and we started again once I felt they had earned my trust.
'Also the fact that he said that other teachers "ignored it" when he put his head on the table leads to me believe that he behaved similarly in other lessons and was not removed and caused no other major problems.' Perhaps, unlike the maths teacher, they had just given up on him.
' "The teacher does not have a problem with the class as a whole, but with one particular student". I am guessing that this was not about one student, but was more likely to be a pattern involving many students, the yawner being another.' I doubt it. As I said earlier, there are always one or two pains in the arse in a class, and all the teachers will moan about them and have the same problems with them.
I am not at all surprised that many teachers thought the OP's son was being rude and should have been sanctioned. We also do not 'fall hook, line and sinker' for a policy. We do this every day so we know what works and what doesn't. I am astounded that you consider rebellious and disruptive behaviour to be the norm and acceptable in school. I certainly don't. Part of what school is about is preparation for the world outside the school gates and learning that one's behaviour has to be moderated to the set of circumstances one is in. If the student still can't moderate or control their behaviour post GCSE or A level, what chance have they of getting or holding down a job? It'll be no use running home to Mummy and complain that their boss isn't being fair when he puts them on a warning for putting their head down on the table when they should be working.
If behaviour is unacceptable the student is warned, and if it reoccurs is sanctioned until the behaviour stops or the student is removed. This allows the other students to exercise their right to learn, and the teacher to exercise their right to teach, which in the final analysis, is the whole point of a school.