Dick Moffat, 50, told GCSE and A-level students at Deben High School in Felixstowe, Suffolk, that when he retires in July he will continue to teach them for seven hours a week until their exams in summer 1998.
However, he has been told that a new government ruling on teachers' pensions means he cannot do paid work at Deben after his official retirement date, even though he could do so at another school.
"I feel a very strong obligation to two groups of students who are unnecessarily having to face possibly disturbing change eight months before crucial examinations," said Mr Moffat, who has taught at Deben for 17 years and is head of English.
"I believe the school would wish me to fulfil the obligation, and I feel the students would too.
"I am offering my services to those classes until their examination deadline without expecting any remuneration. I assured the students in good faith that I would be able to complete their respective courses, fully believing that I would receive a part-time contract for some seven hours a week. I cannot betray their trust, and I cannot shrink from my obligation. If only the same degrees of trust and obligation were offered to teachers generally by government, as opposed to the assumption that we are trying to take advantage."
The Government clampdown on early retirement forced many 50-year-old teachers to retire this year or wait until they were 60.
Mr Moffat, who earned Pounds 23,514 as a full-time teacher, wrote to Education and Employment Secretary Gillian Shephard about his decision. He had planned to teach until 1999.
Headteacher Michael Matthews said: "Dick Moffatt is a gifted teacher who has been disillusioned and discouraged by the changes in recent years. I am dismayed by a system which penalises schools and teachers through the retirement scheme."