Dan Roberts had hoped to become a pilot, but was rejected by the RAF because his legs were too long and then missed out on working for British Airways after it cancelled a training course.
Mr Roberts, 26, said he was still considering other jobs when he began teacher training.
But, after three years teaching science in Plymouth, he is now so enthusiastic about his job that he has spent the past fortnight running a summer school on space exploration.
Mr Roberts, who becomes head of the sixth form at Tamarside community college next term, said: "I fell into teaching because it seemed I had no other option. The attraction was really the financial security - the training salary, golden hello and repayment of loans.
"People tut-tut when I tell them that, but I'm extremely passionate about my job now. I believe the way I fell into the career has been to my advantage because I have had realistic expectations."