As part of my job at a school I received a verbally agreed discount off my child's school fees. I became unhappy with some of the changes at school and decided to withdraw him. I was told that if I did that, then I would no longer have a job - the contracts were renewed yearly. 2 weeks before the end of term I gave a term's notice for my son and gave a 6 week notice for myself(my contract did not have a notice period). After the break (4 weeks later) I recieved the invoice for my son including my staff discount for that last term. I assumed that as I was working some of that last term that they had kindly applied the discount to the whole term. I went back and worked the last 2 weeks of my notice. On my last day I paid in full that invoice for the summer term for my son. I have now been contacted by the school saying they made a mistake in applying that discount and that the invoice had been prepared before I handed in my notice. They have sent 2 letter (which I ignored) and have now passed it onto debt collectors. How should I approach this please? Any helpful advice appreciated.