Dull tax rules:
It cannot be made exempt unless the European Union agrees it - that's never going to happen. Most EU countries charge Vat on children's clothes and are happy to do this.
The UK cannot bring any larger clothing into the zero rate vat rules. The current rules were agreed in 1973 when UK joined what is now the EU, and the UK cannot extend these rules, only restrict them if they choose to.
If you want to start lobbying, the only possible chance of success you could have would be to campaign to bring vat down to 5% on branded larger size school uniform. Unbranded shirts, skirts, trousers and shoes wouldn't be included as would be open to widespread abuse. However, - and it is a possibility - this type of campaign could end up with the government in power making ALL branded uniform 5% vat to avoid confusion for the industry, meaning the current zero rate on smaller sized branded school uniform is abandoned.
On balance, I think the current system, despite its flaws, is the least worst option so I am going to say with a heavy heart, yabu.
And for the record, yes, I am an absolute hoot at dinner parties 