Exactly @thing47 when you say "Perhaps they couldn't afford all the books?"
When I was at school I sometimes couldn't do the homework set as I didn't have the textbook as it was shared between multiple pupils. For history one boy who was in our group lost the particular text for that term and then we didn't even have it in class let alone for homework. My Dad tried to buy the odd book to make up for it but we didn't have a bottomless pit and my bag often got pilfered from so I couldn't even take anything of remote value into school. I got a job as early as I could at 14 and a fair bit of what I earned went on books but it really wasn't a level playing field. I spent a lot of time in the public library (school had turned library into a computer room the year I started at the school). I hope that most schools now don't have experiences where books are in short supply.
I didn't know private schools existed until I went to university and had odd conversations (mentioned up thread about) where people asked where I had been to school and people looked perplexed as I had a very plummy accent that didn't seem to compute with some people that I came from a very working class background and both my parents left with a 'leaving school certificate' and nothing more.
DD is at a grammar and the parents fund a fair bit of stuff like text books and it has less issues with recruitment of teachers than some of the comprehensives. Ofsted even noted what a superb difference the library made to the learning experience. DD has had a brilliant experience compared to mine.
DD's communications from her (hopeful) college at Cambridge have been amazing and for those with minimal funds there are book grants, subsidised rent, means tested grants (of thousands of pounds a year) and tons more. I do think resources like this help even out any disparities whether the student has been on a bursary at a private school or been