Pupil-teacher ratios have worsened for the first time since the late 1990s, according to new statistics.
The figures also reveal that the number of adult helpers in primary classes is up significantly.
Labour pledged to improve the adult-pupil ratio, following its largely successful first-term attempt to get infant class sizes down to 30.
Official statistics show a rise of 0.1 in both primary and secondary teacher-pupil ratios since last year, to 22.6 and 17 respectively.
The number of adults per pupil has risen in primaries from one to 14.6 to one to 14. Secondary ratios improved by 0.2 to 13.1. Ministers pledged to increase the number of adults in classrooms, as part of workload reforms.
Doug McAvoy, National Union of Teachers general secretary, said: "Improving the number of support staff is part of workload reform. A worsening pupil-teacher ratio is not. The amount of time a teacher can give an individual child is reduced with every additional pupil."