Nicholas Eggington was found guilty of two counts of indecent assault and two counts of gross indecency against a male child between 1992 and 1993.
The 58-year-old, who taught at the Cheam School in Hampshire, was placed on the sex offenders' register for life.
He is also subject to a sexual harm prevention order.
Winchester Crown Court heard Eggington, from Nottingham, worked at the school in Headley between 1987 and 1993.
Following sentencing on Friday, a Hampshire Police spokesman said the victim had shown "courage" during the "difficult investigation".

Wales lock Jake Ball is also back along with Scotland back-rower John Barclay.
It is only the second time this season for Davies to partner Williams in Scarlets' midfield.
Former Wales and Scarlets scrum-half Mike Phillips returns to west Wales with Sale, as does lock Jonathan Mills.
The Sharks make three changes with a first start of the season for full-back Mike Haley, and Dan Mugford coming in at fly-half and Ross Harrison at prop.
Sale lie seventh in the English Premiership with two wins in six games while Scarlets are also seventh in the Pro12 with three wins in their six matches.
Holders Saracens travel to deposed champions Toulon earlier in the day in the same group.
Scarlets head coach Wayne Pivac: "What is clear is we're in a very tough pool, but having said that, the majority of them are pretty tough.
"I think it's a good thing from a players' point of view, if I was a player out there I'd want to pit myself against some of the best.
"We'll be going out giving it everything we've got to get a result.
"We'll worry about the other games when they come along. The home games are must wins if you want to get out of the pool."
Sale director of rugby Steve Diamond: " We're probably two or three points behind where I thought we would be at the break.
"We worked hard for the whole of last year to qualify and it is a fantastic group I think. We are up and down at a team at the minute and we need to get some consistency.
"I think I've been there five times as a player and coach and lost five times, it is a difficult place to go as they are a massive club.
"It will be a big test for us as I think we're about equal as teams."
Scarlets: Liam Williams; DTH van der Merwe, Jonathan Davies, Scott Williams, Steff Evans; Rhys Patchell, Gareth Davies; Wyn Jones, Ken Owens (capt), Samson Lee, Jake Ball, David Bulbring, Aaron Shingler, James Davies, John Barclay.
Replacements: Ryan Elias, Luke Garrett, Werner Kruger, Lewis Rawlins, Will Boyde, Jonathan Evans, Dan Jones, Hadleigh Parkes.
Sale Sharks: Mike Haley; Byron McGuigan, Will Addison, Sam James, Paolo Odogwu; Dan Mugford, Mike Phillips; Ross Harrison, Neil Briggs, Brian Mujati, Bryn Evans, Jonathan Mills, Cameron Neild, David Seymour, Josh Beaumont (capt).
Replacements: James Flynn, Halani Aulika, Andrei Ostrikov, Magnus Lund, Tom Curry, James Mitchell, Sam Bedlow, Nev Edwards.
Referee: Romain Poite (France)
Assistant referees: Pierre Brousset (France), Mathieu Noirot (France)
TMO: Eric Gauzins (France)
Citing commissioner: Peter Ferguson (Ireland)
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A teacher has been jailed for three years for sexually abusing a boy at a preparatory school.
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Jonathan Davies returns to partner fellow Wales international Scott Williams in Scarlets' midfield in their European Champions Cup Pool 3 opener against Sale.