. "Everybody feels under stress - the teaching staff, the [school] senior management, the awarding bodies - and at the bottom of the pile are the students. You can't treat them as guinea pigs.
"There are clashes and there have been continual issues over accommodation, and we see on the ground how that impacts on students. Results could be adversely affected. I think we will see it in the statistics."
Exams watchdog Ofqual wrote to schools last week, warning them to expect variations in results because of changes to the structure, timing and content of some exams. Ofqual's chief regulator Glenys Stacey has also said that "it will not be an easy summer".
But the concerns expressed this week are aside from the academic and teaching challenges posed by exam reform. They centre on the practical difficulties of administering 15.5 million exam scripts at once, as a result of the decision to end January and March sittings, reduce resits and move to end-of-course assessment only.
Russell Hobby, general secretary of the NAHT headteachers' union, said he was concerned that the switch to summer-only exams would lead to mistakes.
"There have been cases in the past of administrative errors, and I think that risk rises under the new arrangements," he said. "Although Ofqual and ministers think a lot about the rigour of the papers, they don't think a lot about the rigour of the system of examinations.
"They don't think about the capacity of schools and other organisations to implement it.It seems to me that it is quite important."
The Headmasters' and Headmistresses' Conference, which represents elite independent schools, has similar fears. Peter Hamilton, chair of the group's academic policy committee, said the exam system was already at capacity last summer.
"Now there are far larger numbers sitting at any one time," said Mr Hamilton, headmaster of the Haberdashers' Aske's Boys' School in Hertfordshire. "The honest question is, can the system cope with it? I would be surprised if we didn't have at least as many problems as last year and probably more."
Timetable difficulties were being compounded by tactics such as schools entering students for IGCSE and GCSE English at the same time or having students sit the same GCSE subjects with multiple exam boards, Mr Harland said.
He added that he had been contacted by at least one school a day at the start of exam season, reporting that its exam officer was absent because of sickness or stress. "That will have an impact on students doing their exams," he said. "Are we doing them justice?"
Mr Harland's association had lobbied for the retention of the November and March exam slots, but the government and Ofqual had not "bothered to listen", he said.
A spokesperson for the AQA exam board said it was not aware of a "significant increase in problems" this year.
Edexcel acknowledged that schools were dealing with more candidates. "This year's timetable is in fact very similar to last year's, but it may feel more congested due to fewer students sitting exams earlier in the year," the exam board said.
An Ofqual spokesman defended the switch to end-of-course exams, saying it would free up time for teaching that was previously used for repeated assessments.
"Our approach to awarding aims to make sure that, overall, students are not disadvantaged or advantaged because of changes made to the exams," he said. "We have found that many students were taking a similar number of exams in the summer in previous years, but as resit opportunities."
A spokeswoman for the Department for Education said: "We are scrapping modules and January assessments to put an end to the resit culture, allow more time for teaching and learning and ensure pupils develop an in-depth and lasting understanding of a subject."