Schools are public services and they receive money from the government. When a school purchases anything, they have to pay VAT on it, which goes back to the government, so effectively, whatever money the government gives to schools, between 13.5% and 23% of it goes back to the government, which makes very little sense especially when one considers that the Department of Education only fund primary schools to about 70% of their running costs.
Paying VAT also adds complexity to schools that get any building works done. Schools are now responsible for RCT, which is another burden that already untrained people are asked to do all the time.
If schools were exempt from VAT, it would simplify everything and schools could get more bang for their buck.