Well, imo YANBU OP. I am a teacher and I wouldn't mind doing this at all. However, this time of year, as others have mentioned, is totally mad and stressy, and I have moaned also about colleagues who couldn't tick 5 boxes on doodle! But some people feel the stress more or are under more stress due to covering staff sickness etc and it is pretty overwhelming.
However, as a parent, it is insanely annoying for teachers to have problems with your dc and them not raise it with you. Like you said, if you knew, then you could do something about it and actually support the school and address your son's behaviour. You can't do anything if you don't know.
I have 2 older teens. 1 of them is working his way through the "How to be an Outrageously Rebellious Teen" tick list. Quite successfully
. I go to a PA, and I hear all manner of hair raising things he's been up to (or not) and while a huge part of me is fuming with ds for being such a bloody nightmare, a huge part of me is also thinking well wtf would you wait until December when there have been issues (that I am completely unaware of) since September? Nearly 4 months! It takes a minute to write an email or pick up the phone.
Like you OP, I am totally supportive of the school. I know unfortunately that my son can be amazing, or a total nightmare depending on his mood. But if the parent knows nothing, then you can't be supportive.
Sorry, that was a bit of a rant! OP; I hope that despite her comments, your son gets his grade in Kick Boxing. And maybe the club needs to rethink its system. I get what they are trying to achieve, but a kid can be very difficult in school and highly motivated in sport, and it would be a shame for them to not succeed because of this.