Grades have to be submitted by 18th June by teachers, so I'm assuming that yr11 and yr13s will finish around that time. This hasn't been confirmed yet though.
The last day they need to be at school? Not sure as it might depend on when they have their assessments now and also not too sure if that will be it for Year 11 or if they will have to continue going into school until the end. This year is a mess!