the idea of taking a 15yr old out of private school and handing the school fees to the mum instead is ludicrous.
OP’s partner is already covering all the bases and school fees - anything else would not be for the child, but for the mother who seems to believe her choices should be subsidised.
We have a similar situation - DP’s ex decided to work only reduced hours, despite the children now being 13 and 19. In case of emergency or sickness or anything at all, both DP (and I) wfh and live a 5min walk away, so there are never any issues with childcare or similar. DP pays CMS base plus 50%, all clubs, clothes, driving lessons, pocket money and any other incidentals.
When he left, ex and the children stayed in the mortgage free family home. Ex was not working, so he paid her an extra 1000 a month to cover her own expenses until she found a job - it took her two years. Even after she started her job, he let the extra payments stay for 3 months so that she could save it if she wanted.
So, she is living in mortgage free house, chooses to work reduced hours, he covers all child related expenses, even paid for her and the children to go on holiday the first two years post breakup, yet she tells people (including the children) he left her in penury and they live hand to mouth.