Oh OP and @fedup2017, we are in the same boat. My previously high achieving Year 9 DS is really struggling with his MH and motivation and school work has really slipped.
Initially I was torn, (and I still am a little), but having taken advice from people who have experience in this type of thing I've backed right off pushing hard for school work to be done.
I'm not just doing whatever I can to get DS through this lockdown. I'm trying to get him to do core subjects when he can, and I'm not pushing for good marks, or reacting to poor performance.
The pastoral team at school actually suggested that all school pressure is eased and have made sure his teachers are aware. That has really helped.
I know it's incredibly hard to know what to do. There's a background worry that you are being manipulated. Being firm, having standards, working hard have previously been my 'go to' approaches- but right now, love, compassion, understanding, and patience seem to be what is needed. I can't risk alienating DS further, and so I need him to know I'm on his side and we are in this together.
The conversation about moving sets sounds way out of line to me. School should be way way more understanding than this.