Rise of the supersized schools: Baby boom and immigration behind increase in primaries with more than 1,000 pupils Number of schools with more than 700 pupils up 60 per cent in three years Headteachers 'forced to open classrooms in playgrounds and libraries' The size of primary schools has rocketed in the last three years as Britain deals with an explosion in the birth rate fuelled by the rise in young immigrant families. There are 60 per cent more supersize primaries with more than 700 pupils - including some with more than 1,000 - than in 2010, according to Department for Education statistics. While three years ago there were no schools with more than 1,000 pupils, it is now becoming more common to have six classes in each year. Supersized: The number of schools with more than 700 pupils has jumped by 60 per cent in three years There are now 130 schools with more than 700 pupils, compared with just 80 three years ago. Supersized schools are most prevalent in deprived areas, particularly east London and central Birmingham, where cheap council housing is available for families. Barclay Primary School in Leyton, east London, already has 1,200 pupils and will expand to accommodate 1,600 by 2014, according to the Guardian. Local authorities are forcing headteachers to open mobile classrooms on playing fields and in playgrounds, music rooms and libraries, in order to comply with laws meaning councils must find all children a school place, the newspaper reported. But a National Union of Teachers study earlier this year found a fifth of areas where new free schools were being built, under the Government's scheme, already had at least 10 per cent of places going spare. Colin Ross, a school governor and Sheffield city council shadow cabinet member, said primary schools should not exceed 420 pupils - or two classes of 30 in each year. He told the Guardian: 'Parents want to know that primary school teachers know their children. If a school becomes bigger than 420, it is very difficult for staff to know each child. Loss of space: Some headteachers have been forced to open extra classrooms in playgrounds and on playing fields Bob Garton, head of the 1,200-pupil Gascoigne Primary in Barking, east London, said: 'We have no open space. We had a playing field, but temporary classrooms are on that now. We don't have one spare room. We are full to bursting. But Kay Jones, headteacher of Pinkwell primary in Hayes, Middlesex, which currently has 983 pupils and will expand to 1,200 by 2016, said size was not a barrier to good teaching. 'Class sizes are the same as in other smaller schools and we make sure there are only 300 children in the playground at any one time,' she said. Studies have shown that pupils may be less likely to be bullied in larger schools but research on whether large primaries are better or worse for children are inconclusive.