The teacher should have told your son off, told you and told the other parent it was dealt with.
I cannot imagine what the other parent thinks they are going to say to add to the situation and why the teacher has suggested it, UNLESS it is about something extra which hadnt been made clear, ie teacher said I've sorted it out but I dont know about xxx you'll need to ask mrssoap about that herself. It might not be entirely related. That's the only thing I can imagine. If it's not i would take it up with the teacher, why she is unprofessional handling the matter and point out its unacceptable
As others have posted I really recommend a daily communication book. You write in significant events from home like DS went to bed late, was naughty, or had an early night, enjoyed the picture homework etc and they write back about things throughout the day it helps everyone get a better all round picture of the child when there are definite struggles.
I know how hard it is when you are battling with a child with school struggles, and although you have commitments to your older childrens activities, for a better family health it might be worth having a terms change where you try a different tactic. All activities end earlier, family dinner, bed is earlier, as much of a structured routine by time as possible is implemented and see what happens. Ie set times for everything.
It could be your son is struggling with lack of obvious routine and whilst its manageable at home with a small environment, he is low level tired and in a big scale noisy class room it's too much. His inability to cope is coming out as disruptiviness. Its not uncommon in children your sons age they dont have coping mechanisms or even the understanding of what's wrong. They try and "escape" the situation however they can, disruption is an easy way.
As others have said your child may also for any one of a number of reasons have sensitivities to noise, order(or lack of)movement and again be unable to cope. For some kids the world is just a bit too much. School should definitely be looking for ways to discover a possible cause so the problem can be addressed and you can move forward with a much easier life.
You have other kids, you're doing your best and implementing the best strategies you've found. If you have a fuller picture you may well be in a so much better position to move forward on a more stable footing.
The teacher thing is definitely wrong and she should be told, but perhaps this is actually something in a weird way that might indirectly be able to help your overall situation? There are lots of knowledgable people on here so maybe give it a try. First and foremost get that communication book from school.... And have a great christmas