The start of work on Banbury School's new £6m classroom block was marked by the laying of a foundation stone by MP Tony Baldry.
Hornton Quarries donated the stone and Mr Baldry paid for it to be engraved.
As well as new classrooms, the block will include a dining hall, common room and offices.
The existing Stanbridge Hall building will be sold to help fund the new building, though about 90 per cent of the total will come from the county council.
The new classrooms were first announced September 2006 after a long campaign by school principal Dr Fiona Hammans for better facilities.
She had called for a fairer system of investment in education - saying that Drayton School was getting millions of pounds of private cash to turn it into an academy, while her school was getting nothing.
Dr Hammans and Banbury School are now faced with raising £500,000 to furnish and equip the new buildings - and are appealing to the wider community for donations.
The new classrooms will be completed by November.
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