A £700m invoice will be handed to the Department for Education this afternoon by campaigners.
The National Education Union says that's what's needed to properly fund English sixth form colleges.
Workers from 25 of them including Hereford Sixth Form College are marching through central London as they begin a strike to demand more money.
NEU says its members are taking action to "secure the funding needed to sustain fair pay, conditions and employment including reversing job losses, class size increases and cuts to teaching time and curriculum provision."
Education Minister Michelle Donelan said it was "very disappointing" that the NEU had decided to take strike action.
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