I have not looked at the detail, but this would for example:
School has 1000 pupils and receives £500,000. Equates to £500 per pupil.
School has to accommodate an extra 18 pupils yet still receives £500,000 (ie no budget cut). Equates to £491 per pupil.
Actually, i misread it - 9% cuts and -£286 per pupil by 2022 under the tories, along with 4 less teachers. The methodology is very vague on that website, based on "sensitive information which is why it is not generally available".