My dd is now in Y11 and doing very well (thanks largely to the fabulous teacher she had for Y3 and Y4, who protected her from the class bully, inspired her in her work ethic and gave her self-confidence). This year dd can attend school until 5pm every day if she wants to, to do extra work in any GCSE subject she feels she needs a bit of help or practise on, or just wants to spend a bit of extra time on because she likes it.
Every GCSE subject has at least one teacher teaching until 5pm; there are also extra sessions at lunchtimes. This is in case a pupil wants some extra help. The teachers don't have to do it, I'm sure it's not in their contracts that they have to do extra teaching every week for Y11s. They do it because they want their pupils to achieve the best they can.
They're not doing it this year only, either. Last year they did it for those Y11s, and next year, they'll do it for those Y11s, and at some point they'll do it for your child, so every single year they are teaching until 5pm despite school finishing at 3:40. As someone said upthread, these are the people who are enabling your child to become a doctor, or a lawyer, or a vet or a nurse or even a teacher!
Yes, teachers are bloody marvellous.