All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
According to jurisprudential adjustment, schools' canteens are based on the contracts of partnership and speculation; whereas the contributors subscribe in the capital, then they authorize someone to run the canteen and the profits should be specified as a given percentage and known in the contract.
Therefore, a speculator is obliged to be keen on the interest of the person who authorizes him for the Prophet (PBUH) said (What means):" All of you are guardians and are responsible for your wards." [Related by Al-Bukhary]. Thus, if the person responsible for the canteen took bonuses or accepted gifts during job, then this is considered an abuse of trust as well they are ill-gotten money. In addition, the degree of prohibition increases if he raised the price of good for his own benefit, since Allah, The Exalted Said (What means): " O ye who believe! Squander not your wealth among yourselves in vanity," [An-Nissa/29].
In conclusion, whoever took anything unlawfully out of the canteen is obliged to give it back, as well the speculator should fear Allah and be reliable with the person who hired or authorized him in order to free himself from the liability before Allah. And Allah knows best.