I’m not a teacher but I work for the NHS and one year I was due a back pay and I literally got £10 more. The tax, NI, pension literally took it all. I questioned it and they said it was correct, so not sure if it’s the same.
I’m a teacher, and when receiving back pay in previous pay rises the amount of tax/ NI/ pension contributions that has been taken has been ridiculous! You can end up not really receiving much more or sometimes less
I have had this with work recently - were you on maternity when you are due the back pay? If so you basically don’t get it and then mr tax man takes most of it anyway. Obviously get it checked but I basically missed out on the back pay because of SMP. I ended up with £200 extra this month and that was from a pay rise and 4 month backpay 😭 my payslip is a confusing mess too but definitely get on to HR and figure out what it all means.
I’d contact payroll yourself instead of letting school sort it. You’ll get it sorted quicker xx