Although he funded it himself and studied in his own time, he nevertheless regarded the Masters as vital for his professional development.
After months of lengthy correspondence he got an answer, though not the one he would have wanted. Yes, an employee could receive tax relief for a course regarded as improving professional effectiveness, but it had to be full-time.
Those paying their own way through the National Professional Qualification for Headteachers face the same bar to tax relief, the Revenue told Mr Carroll. Given that a Masters degree, and an MEd in particular, is often looked upon as a crucial qualification for those seeking school management posts, the lack of financial help is iniquitous, says Mr Carroll.
"I don't think the Inland Revenue has thought about the particular circumstances of teachers and others who are not given time out of work. If they have then this is a poor answer. If they have not, they should go away and think again. This is an equal opportunities issue. Teachers are not given the same chances as many other professionals."
Mr Carroll had not sought funding for his MEd from his school, knowing it could not afford it, but he had expected to be able to claim tax relief.
"Teachers are paying through the nose for these courses, squeezing the work into weekends and evenings because they know they will improve professionally, " he says. "They are not swanning about doing origami. I don't think it's unreasonable to ask for a little bit of help."
Currently, only 26 of the 680 new Masters students at London's Institute of Education are funded wholly or partly by their schools. The institute lays on evening classes especially to help teachers who have to study out of school hours.
Michael McGarvie, head of the institute's further professional development section, says that while applications for Masters degrees have increased because they are seen as an essential stepping stone to promotion, funding has dried up. Up to 10 years ago teachers might have been given a year's secondment through their local education authority to complete an MEd.
Geoff Holman, assistant secretary at the National Association of Head Teachers, says that an MEd is "becoming more and more of an expectation, but how and when teachers do it has become entirely their own business".
Graduate recruiters for other jobs see professional development opportunities as crucial. Chartered accountants Arthur Anderson, for example, offers employees who have worked with the firm for four years the opportunity of studying for a fully-funded MBA at the Manchester or Warwick business schools.
Alongside such incentives, the opportunities offered to teachers seem paltry and potentially disastrous at a time when the difficulty of recruiting graduates is becoming critical.
The Department for Education and Employment declined to specify what funds were made available to its own employees for professional development, though the Teacher Training Agency said that this year #163;60,000 had been provided for its 100 staff. "I think any head with 100 staff would be more than happy with that amount," says Geoff Holman.