I'm so sorry - we had a similar response earlier in the year from our DS6's teacher. We'd been having just awful behaviour problems for several months, and her advice was that he obviously would therefore need to stay in his allocated grade to practise social skills. And of course team sports would be wonderfully beneficial.
We didn't argue too much - just left our paperwork and kept a low profile. I appreciate now how lucky we are that we have departmental policy behind us, no matter how unwilling teachers might be. We didn't actually mention it, but they and we know that we've got it behind us. So a little while on, we haven't heard much from the school, but DS6 seems to be working a grade up. And the behaviour problems have stopped, really quite suddenly. We'll raise our heads again soon and talk to them about formalising the arrangement.
I can't offer any advice either, just sympathy and the hope that you find a quick path through this.