Revision sessions by school are usually the first week of Easter holidays. There is no way Ds would have given up an opportunity to study. Dropping a few marks on each paper does affect grades. As Ds's friends found on on results day. Maybe have a look at grade boundaries to see how tight some of them were.
I wouldn't want to put my children in a position of wondering if they hadn't gone on holiday would their grade be higher?
Easter is an opportunity to revise for exams that fall before May half term, so in Ds's case
2 papers for English Lit,
2 papers for Computer Science,
3 paper 1s for science (Biology, Chemistry, Physics)
1 maths paper.
May half term is smack bang in the middle of GCSEs and an opportunity to revise for the exams coming up afterwards. Which were
3 history papers,
2 maths papers,
3 science papers
3 German papers,
2 English Lang papers
2 Statistics papers.
Maybe when you see it laid out like that it might make you see how many exams they do. And yes, Ds did achieve incredible grades, but he worked for them.
Ds then enjoyed 10 weeks off school from the end of GCSEs until he started sixth form. That is the time for a holiday.