A school has raised £1.7 million to pay for new buildings.
Teachers, pupils and parents from Worth School, near Crawley, raised the cash 15 months since they started, and are just short of their £2 million target.
The money is being used to fund building work at the all-boys school.
But although official fund-raising stopped at the end of March, headteacher Peter Armstrong said they were "optimistic" they could still reach their target.
Headteacher Peter Armstrong said: "We have raised £1.7 million so far and the appeal has part-funded the current school development plan, which includes the building of the new Upper Sixth House, enhancement of sports pitches plus the renovation of the boarding houses.
Mr Armstrong added one of the boarding houses in the main building was currently being refurbished and would accommodate Rutherford House after the summer for 70 students aged 13 to 17.
During the summer work will begin on refurbishing Butler House, which is the main building.
Father Christopher Jamison, the present Abbot of Worth Abbey and former headteacher, said: "I am often asked why somebody should offer financial support to Worth as well as to charities that fight disease to help less developed countries.
"The answer is literally everybody is a potential donor to such charities whereas Worth is dependent on those who know us personally and value our work.
"Our circle of support is as small as we are and therefore every one of our friends is a vital contributor.
"For our distinctive education to flourish we must have resources, both physical and spiritual, which is why we need to complete our current school development plan.
"To avoid serious delays to the building programme I urge everybody to contribute to the appeal as soon as possible."
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