Peter Jackson, 36, over-charged wealthy parents for a school expedition to Tanzania and Zanzibar that he had organised in July 1996.
Wood Green Crown Court, in north London, where Jackson will be sentenced on Friday,heard how had he spent "vast sums" on his own children's private school fees and piano lessons, his mortgage, and renovations including a new thatched roof at his country cottage.
After the jury delivered unanimous guilty verdicts on 12 sample counts of theft and one of forgery between April 1995 and November 1996, Judge Leonard Gerber warned Jackson that prison was almost certainly the only option.
In his defence Jackson, of Charminster, Dorset, had argued that there was a "culture" at Harrow that allowed him and colleagues to operate as one-man travel companies, and profit for the time and effort involved in organising overseas expeditions.
The headmaster of Harrow, Nicholas Bomford, said: "One assumed one was dealing with someone one could trust. He slipped through the net."