Workers have one week to transform a building site into a £14 million school.
Piles of rubble and building materials litter the car park of the new St Paul's Catholic College in Burgess Hill.
The grounds have yet to be landscaped and windows remain without glass.
The new campus was scheduled to open up to its 800 pupils next Monday but that has now been put back until Thursday due to last-minute snags in the building work.
Lessons will eventually begin up to a fortnight after many schools in Sussex.
Parents were told of the unexpected extension to the summer holiday by mail.
Construction firm Gleeson began work on the new campus 18 months ago.
The school was reluctant to discuss the situation yesterday and refused to explain the cause of the delay.
Headteacher John Flower said: "The parents have been fully informed of the progress of the project throughout the summer and it's a testament to Gleeson's hard work that teaching will commence only three days later than anticipated. Our parents realise that to wait for a £14 million school for three extra days is a very small price to pay.
"All our parents and students are fully informed of the induction process and can't wait to enjoy our fantastic new campus."
The finished building will include the latest computer technology, a sixth-form centre, sports hall, drama studio and extensive playing fields.
It will also harness alternative energy sources such as wind and solar power.
The project has been funded partly from the sale of the Haywards Heath site and partly from Government grant.
Neither the school nor Gleeson would say when work was expected to be completed. It is believed some construction will continue until at least half-term.
St Paul's is one of the top-achieving schools in the mid-Sussex area. An impressive 70 per cent of pupils gained five or more GCSEs and grades A*-C this year.
The school outgrew its Sixties campus in Haywards Heath and the new college is being built on farmland opposite Triangle Leisure Centre in Jane Murray Way, Burgess Hill.
A subway will connect the school with the leisure centre.
The Arundel and Brighton Catholic Diocese, a partner in the school, declined to comment.
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