By Angela Harrison
BBC News, at the ASCL conference
Rising exam costs mean some schools are spending more on putting pupils through exams than on books, head teachers say.
Schools surveyed spent about 2% of their budgets on exams
Pupils are sitting more exams and they are getting more expensive, according to the Association of School and College Leaders, meeting in London.
A study of some schools found a 51% rise in total fees over the past three years, they report.
The union warns the situation will get worse next year with the introduction of new vocational Diplomas.
ASCL president, Malcolm Trobe, told delegates: "The schools in the sample were spending an average of nearly 2% of their school budget on examination fees. One school was already at 3%".
"Many schools reported spending more on examination fees than on subject capitation - books, photocopying and other learning resources."
He said part of the reason was the increase in modular exams - pupils were taking more exams so the costs were higher.
Also, the exam boards - which are non-profit-making - were spending money developing new qualifications and were probably passing on those costs, he said.
Head teacher of Bexleyheath School in Kent, Malcolm Noble, said his school's bill for exams was definitely rising.
"When I first came into my current job 14 years ago, the budget for examination entries was not insignificant but not huge.
"Now it's hugely significant. It's a huge and inescapable cost."
At his school of 2,200 pupils including a sixth form, the bill for exam fees last year was £150,000. The amount spent on books and other resources related to the curriculum was £260,000.
The head of King James School in Knaresborough, Carole Walton, agrees fees are rising.
"The bill for exams has gone up significantly. We are paying more and at the same time teachers are doing more marking because of the coursework," she said.