Thousands of five- to seven-year-olds are still being taught illegally in classes of more than 30, according to Government figures released yesterday.

An annual census of state schools published by the Department for Children, Schools and Families revealed 4,280 key stage 1 pupils were being taught in 130 oversized classes, contrary to legal requirements.

A further 18,930 schoolchildren aged between five and seven were also taught in classes of more than 30, although the schools involved had been granted exemptions to do so, often because they had to admit an emergency pupil in the middle of term.

The figures were an improvement on 2006, when more than 7,000 five- to seven-year-olds were taught illegally.

But teachers' leaders said last night the failure to eradicate oversized classes completely placed a question mark over the priority ministers were giving to the reduction of class sizes, particularly after Gordon Brown made personalised learning the key education policy theme of his first leader's speech to the Labour party conference in Bournemouth.

Steve Sinnott, general secretary of the National Union of Teachers, Britain's largest teaching union, said: "How can you deliver personalised learning in large classes? We thought the implication behind the commitment to reducing class sizes to 30 for infants in 1997 would be a drive to rule them out elsewhere.

"We have to be careful that the foot is not taken off the accelerator in ensuring further reductions. After all, those parents who choose to send their children to independent schools understand the value of smaller class sizes."

Overall, yesterday's figures showed the average size of classes for five- to seven-year-olds remained unchanged at 25.6. For seven- to 11-year-olds, the figure fell slightly to 27.2. The percentage of primary school pupils in large classes fell by almost a percentage point to 14.4 per cent. In secondary schools, the average class size dropped from 21.5 to 21.2 between January 2006 and 2007.

The data was released just one week after an international study by the Organisation for Economic Co-operation and Development showed the gap in average class sizes between state and independent schools was larger in Britain than anywhere else in the Western world.

But Lord Adonis, the Schools minister, said yesterday's figures showed "encouraging progress" had been made on reducing class sizes with the average "falling or remaining broadly stable across the board".

"Smaller class sizes and well-supported teachers are vital if we are to ensure every child gets the individual attention they need to thrive at school – so we want this trend to continue," Lord Adonis added.

"In particular, the proportion of large infant classes has fallen from 29 per cent in 1997 to 1.7 per cent this year and the number of key stage 1 classes that are exceeding 30 pupils without good reason is down to just 130. But it's important this falls further."

The census also showed a significant rise in the number of pupils from ethnic minority groups in state schools – up from 20.6 per cent in primary schools in 2006 to 21.9 per cent this year. In secondary schools the figure increased from 16.8 per cent to 17.7 per cent. The data also revealed that the number of women entering teacher training courses outnumbered men by almost three to one – 23,865 against 8,065.

Meanwhile, separate research showed a major rise in the number of young people opting to take Mandarin at A-level and Chinese studies at university. The number of Chinese studies undergraduates has risen from 1,045 in 2003 to 1,945 in 2006. At A-level, the number of candidates almost doubled from 1,102 in 2001 to 1,996 last year.

But the numbers studying Chinese at GCSE have fallen from 2,124 to 1,827 in the same period, according to the Centre for Information on Language Teaching, which conducted the research.