Under Labour, schools have struggled to attract sufficient teachers to keep pace with an increase in pupils, official figures show.
In secondary schools, an additional 11,200 teachers have had to cope with an extra 222,500 pupils.
But class sizes have risen only slightly in secondaries and have fallen in primaries, suggesting teachers have had to give up free periods and leave tasks such as marking and preparation until after school.
The fresh evidence that teachers are working harder is contained in the latest class-size figures from the Department for Education and Skills.
At secondary level there was an average of 17 pupils for every teacher in England during Labour's first five years in office. This compares to 16.5 during the Tories' last five years and just 15.5 in the late 1980s and early 1990s.
Class sizes fell last year for the second year in a row to 22 pupils, an average of 0.1 higher than in 1997.
In primary schools, there were 23.2 pupils for every teacher over the past five years compared to 22.1 between 1988-92.
The average class now has one pupil fewer than when Labour came to power.
The figures also show that last year there were almost 10,000 five to seven-year-olds in large classes despite Labour's claim to have eradicated classes of more than 30 for that age group.
John Bangs, head of education at the National Union of Teachers, said:
"Non-contact time has been under pressure as heads have tried to make up for higher pupil-teacher ratios. Proper non-contact time in primary schools has been stillborn."
He said increased use of unqualified staff had also helped to keep class sizes down.