One issue in this election seems to hark back to 1997: school class sizes. Then it was one of Labour’s key manifesto pledges to cut class sizes to less than 30 for all five, six and seven year-olds. This was such a major issue for the party that it even featured as one their five pledges on their election "pledge cards".

Now the Liberal Democrats have returned to the issue. In a recent article in The Independent, Nick Clegg claims: “Thousands of children are in classes so big they're technically illegal.”

This is a powerful statement but what do the statistics tell us? First let’s look at the legal side. The Government put its 1997 pledge on a statutory footing in 1998 when it made it a requirement for all Local Education Authorities (LEAs) and schools to restrict Key Stage One class sizes (ages 4 to 7) to 30 pupils by September 2001.

Data from the 2009 School Census conducted by the Department for Children Schools and Families (DCSF) presents a slightly complicated case. (Please note that the following data refers only to state and maintained schools in England).

The DCSF states that in 2009, 930 Key Stage One (KS1) classes had more than 30 pupils. That figure is out of a total of 53,410 classes. By this measure, there are certainly “thousands” of children in classes that are technically illegal in size. However, the DCSF makes a distinction between KS1 classes with over 30 pupils and KS1 classes reported as “unlawfully” having more than 30 pupils. Of the unlawful type, there were 310. Therefore, a relatively conservative estimate of every such class having around 35 pupils would still put the number of pupils in just plain illegal classes well into the thousands.

The distinction between classes lawfully and unlawfully over 30 pupils is based mainly on a few cases where exceptions are made. Of KS1 classes lawfully over 30 pupils, 53.2 per cent were granted as exceptions because of a review from independent appeals panels or where pupils were initially refused entry because of errors.The second biggest reason why classes over 30 were not deemed unlawful (21.6 per cent of cases) was the need to accommodate children admitted outside of the normal admission rounds. That said, why exactly the other 25.2 per cent of classes are over the legal limit without being unlawful is not entirely clear.

The Government seems to have made very little progress in reducing primary school class sizes in recent years as the following data reflects. Although reductions have been made in the proportion of large classes and the proportion of students in large classes, they are hardly substantial.  

 
An important point about Mr Clegg’s "technically illegal" criticism is that the legal restrictions don’t even apply to secondary schools. The proportion of large classes and proportion of students in large classes are both lower than among their younger counterparts, but have shown little change in recent years. 
 
 
 
Although there’s debate about the importance of class sizes, an OECD report in 2008 found that the average class size in Britain (24.5 at the time of publication) was “very large”. The report also criticised the large gap between state and private schools – private schools had an average class size of 13 when the report was published. The following graph show’ Labour’s long-term performance on class sizes:
 

 
This time round, the LibDems have promised to cut the average primary school class size to 20. Mr Clegg's claims about Labour's record, on these statistics, are justified - indeed, for once it seems as if a political issue might just have been understated.
 
It’s not entirely clear why more than 30 students in any class, primary or secondary, would not be considered too many if that is where the limit is set. If it were indeed illegal for students of all ages to be in a class of over 30, there would not be "thousands" of children in classes that are technically illegal but hundreds of thousands, well over half a million.
 
There were in fact 202,237 secondary school students in classes with over 30 pupils in 2009. The same figure for primary school students was 457,045.
 
In education policy, there isn’t much else that can cause such a stir. Parents in particular perceive class sizes to be extremely important – a MORI survey found that his was the biggest factor for 36 per cent of parents sending their children to private schools.
 
Given Labour’s relative inability to bring any substantial changes to class sizes since 2002, it will be interesting to see if in a time of such austerity - and should the results of the election provide them the opportunity - the LibDems can meet their pledge.