A Leeds primary with an infant class of 45 could become a test case to establish whether class-size legislation takes precedence over workforce reform.
Reception pupils at Deighton Gates primary, in Wetherby, are registered in two groups, but most of their lessons are taught by one qualified teacher and a team of four support staff.
The 1998 Act which imposed a statutory upper limit of 30 pupils in infant classes is now a "grey area", said Jeremy Dunford, the head. It conflicts with the school workforce agreement and the foundation-stage curriculum, he told colleagues at a workforce reform conference organised by Leeds and Bradford education authorities last month.
The plan has been approved by Education Leeds, the public-private partnership which has run most education services in the city since 2001.
But the Department for Education and Skills said it will now investigate whether the school is breaking class-size rules.
Teachers' unions have condemned Mr Dunford's strategy.
The 1998 law states that no more than 30 infant pupils should be in a class while an "ordinary teaching session is conducted by a single qualified teacher".
Government figures released last month showed that in schools in England there had been a dramatic rise in the number of infant classes with more than 30 pupils - from 65 last year to 530.
Mr Dunford said: "I am playing the game a bit in terms of the legislation, but I am meeting the legal requirements as judged by my LEA."
The plan has led to some parental opposition and resulted in five pupils leaving, Mr Dunford said.
The move was prompted by falling rolls, long-term overstaffing and an unsustainable financial position at the school. In January 2004, it faced a pound;222,000 deficit within three years if it failed to act. But Mr Dunford also cites educational reasons.
"I have enough teachers to split up classes so that there is one per 30 pupils," he said. "But I am not doing that because it would mean having mixed-age classes, which I prefer not to do. Workforce reform is about not having to have qualified teachers there all the time and using other adults."
Mr Dunford said his system, which was praised this year by inspectors from the Office for Standards in Education, was also more suited to the foundation-stage curriculum because it allowed for a more flexible, less formal approach than the "ordinary teaching session" described by the earlier class-size legislation.
John Howarth of the National Union of Teachers in Bradford, who heard Mr Dunford's conference speech, said: "We have fought long and hard for infant class-size limits and the route he is suggesting is totally unacceptable."
But Chris Edwards, chief executive of Education Leeds, said it was a creative solution to the problems of workforce remodelling which did conform to class-size legislation, and that he fully supported it.
A DfES spokeswoman said: "We will investigate any schools that appear to be exceeding the infant class-size limit, this school being one of them."