Tories and Lib Dems: if parents get jobs, schools lose money

I've just noticed that both the Tories and the Lib Dems plan that if a parent of a school age child gets a job, their child's school will lose thousands of pounds in funding.

It's the consequence of the so-called "pupil premium", which allocates extra funding to schools for each disadvantaged child which attends the school. The Tories are vague about how much the premium will be, whereas the Lib Dems have put a figure of £3,000 per pupil. Unless they are planning to set up another multi billion pound IT database to track the incomes of every parent (and what could possibly go wrong with that?), qualification for the premium will presumably be based on eligibility for free school meals.

Hence under the Lib Dem plans, if seven parents get jobs, their children's school loses the equivalent of an entire salary for a newly qualified teacher. I am sure this isn't what they intended, but I am struggling to see how this is going to help tackle educational inequality or improve schools.