A teacher returned to the staffroom after going to a mosque for prayers and found his laptop missing.
He then lodged a police report about the disappearance at Yio Chu Kang Secondary.
This happened in June last year and investigators traced the $2,150 computer to a second-hand shop on Commonwealth Drive.
Its records revealed that the computer had been sold to the store by Asogan Sokkalingam - a relief teacher at the same school.
Yesterday, the 37-year-old was jailed for nine months after admitting that theft and misappropriating another from the school.
A police report was also made by a head of department in January this year, saying that Asogan had not returned a laptop loaned to him to conduct lessons - despite several reminders.
It turned out that Asogan had taken the $300 computer with him when he resigned in August and sold it to a shop for $150 as he was facing financial difficulties. Neither laptop was recovered as both had been exported.
Asogan had been jailed nine months for forgery in 2005 and six months in 2010 for criminal breach of trust.
Pleading for leniency, defence counsel Amarjit Singh said that full restitution had been made. He said that Asogan, who is married, "profoundly regrets" committing the offences which also included criminal breach of trust. His sentence will start on Aug 23 as he needed time to settle some personal affairs. He could have been jailed for up to three years and fined up to $10,000 for theft.