Year 7 and 8 pupils at the Priory School in the London borough of Bromley, which is short of six teachers, leave at 2.20pm on Mondays and Tuesdays respectively.
Headteacher Nick Ware has cut the 25-hour teaching week by an hour and is sending pupils home early. Last year he filled the gaps with trainee and supply teachers, but found that discipline suffered and senior staff were overworked.
"None of us likes the situation, but too many unqualified people compromises standards," he said.
Year 9 pupils at the school in Orpington, Kent, leave early on Wednesdays and miss an hour of drama one week and RE the next.
Two-thirds of pupils in Year 10 and 11 miss an hour of non-examined subjects, and leave early on Thursdays and Fridays respectively.
Mr Ware said the recruitment problems are the worst he has seen in his 30-year career and his priority now is to recruit a maths teacher so Years 7 and 8 could return to a full timetable with three hours of maths from January.
"Those who care about education have a responsibility to talk up the value of being a teacher," he said.
Parent governor Nona Firmin says her son Paul has extra maths homework to make up for the missing lesson: "There have been mixed views from parents but Mr Ware's first interest is the children so if he feels they would benefit from this arrangement it is certainly worth trying, as long as they are given work that can be done at home."