How about, "no, sorry, we'll be organising our own gifts, as we do every year". Perhaps without the 'sorry'. It shows you won't be coughing up this year or any year, but without it sounding like it is because you (1) can't afford it, (2) think it is too much, (3) weren't born yesterday, (4) would like to spend more on your own mother than school. Or a mix of all 4.
My niece is at a fee-paying school (we're abtoad, ut isn't private, but it is paid), the extra stuff they have to contribute us incredible. For carnaval, they had to wear a different fancy dress EVERY DAY FOR A WEEK on a different theme: Monday farm animal, Tuesday flowers, Wednesday a uniform... etc. They had to buy black shoes for the Christmas concert because niece didn't have any. For a Christmas concert. She was 3. (No, no Primark nearby). They're constantly pulling ballachey tasks out of their arse for parents.
Having said that, I was at boarding school in the 80s, and bar uniforms and the odd school trip, my parents weren't hit up for anything!
You might find, OP, that if you said "no way, Jose" to the £80, many relieved parents might follow suit.