No - this is appalling. I would start writing to the head about it. With a lot of “as you know”s and “as discussed”.
“As you know, my former husband picks up my child on a xxxxday after swimming, and as a result of our acrimonious divorce, refuses to part with the swimming gear. The practical effect of which is, as I’ve explained, that I have to buy him a new pair of swimming trunks and a new towel every week. I have asked if the school could hold onto his swimming gear until the morning for me, but you have said no.
I am asking you to reconsider. My ex-husband now has (or has disposed of) 10 pairs of swimming trunks. You have intervened on ex-husband’s behalf, with pickup at a different door, so I am unconvinced by your argument that you cannot make special arrangements.
I am again asking you to allow DS to leave his swimming gear with the school office overnight. If you again refuse, I will be raising this with the governors and after that with the local authority / education ombudsman / local MP.