This summer, the total bill at Croesyceiliog comprehensive in Cwmbran, south Wales, was pound;106,726. GCSE pupil numbers have risen, from 247 to 281, but fee increases have been more significant.
In 20023, a GCSE with the Welsh exams board WJEC cost pound;18.50 and an A-level module was pound;9. For the current year, they cost pound;22 and pound;11.50 respectively - up by a fifth or more.
"Our budgets are not inflating at the same level," said Mrs Mansfield, outgoing president of ASCL Cymru. "We are going to have to start eating into resources we have allocated for courses. When that happens, that will have an impact on achievement."
Derec Stockley, the WJEC's director of exams, said: "The cost of securing examiners and moderators is increasing. In some subjects there is a shortage of examiners, and payments have been increased with the help of government subsidies.
"From 2007 the subsidies are being phased out, and it seems very likely that WJEC will have to pass on the extra costs."
The move towards electronic distribution of many exam materials, and the provision of on-screen assessment, could held drive down costs, but the full impact of such changes is uncertain at the moment, he added.