GCSE maths students in a Barnsley school are being taught in a large class of up to 65 pupils.
A worried Barnsley councillor complained to Horizon Community College about the oversized class which has been running for about a month because a teacher is ill.
But headteacher Nick Bowen said the school was making the best use of its most-qualified teachers, and its modern building, which is designed to accommodate occasional larger classes.
Coun Gill Carr said: "I'm very concerned. These are year ten students doing GCSEs. They can't concentrate if they've got 65 in the class."
One parent, who didn't want to be named, said: "There's too many people in the class not wanting to learn. My child does want to learn. I understand they've got a teacher off sick, but they've got to do something about it."
But Mr Bowen said it had been the best move for the pupils.
"This is not just any teacher taking this class. He's an advanced skills teacher. An outstanding teacher.
"It's better to have an outstanding teacher take both classes together, rather than half of them get an outstanding teacher and the other half get a supply teacher just giving out a few worksheets."
He said the school had been designed with special flexible classrooms which could be opened up for larger groups, but that if the ill teacher doesn't return after Christmas, the school will look at alternatives.
He added: "Our maths results are rocketing. This year 63 per cent of students achieved the benchmark five GCSEs at A* to C including maths and English - higher than the national average.
"I think there was only us and Penistone achieved higher than the national average."