All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
According to its legal characterization/classification, school cafeteria is a partnership and Mudarabah where shareholders contribute capital, then appoint an agent to manage the affairs of the business. It is worth adding that the profits are divided by ratio. The Mudarib (working partner) is the agent that rabb-ul-mal ("silent partner", "financier") has consented to run the business, so he is trustworthy and must preserve the best interest of the authorizer (rabb-ul-mal ). The Prophet (Peace and blessings be upon him) said, "All of you are guardians and are responsible for your wards. The ruler is a guardian and the man is a guardian of his family; the lady is a guardian and is responsible for her husband's house and his offspring; and so all of you are guardians and are responsible for your wards." {Related by Bukhari}.
All the actions of the agent must observe the best interest of the authorizer to the fullest extent possible. Imam Shihab Ar-Ramli (May Allah have mercy on him) said: "A partner is like an agent in terms of running the business in the best interest of the authorizer." {Hasheyat Ar-Ramli Ala Asna Al-Matalib, Vol.2:P.257}.
It is impermissible for the person appointed in charge of the school cafeteria to take the bonuses offered by the traders of food products or accept gifts to his personal gain since if he does so then he betrays trust and eats up others wealth unlawfully. Moreover, if he raises the prices of the products sold to the students, then he incurs a greater sin since Allah the Almighty says {What means}: " O ye who believe! Eat not up your property among yourselves in vanities: But let there be amongst you Traffic and trade by mutual good-will: Nor kill (or destroy) yourselves: for verily God hath been to you Most Merciful!" {An-Nisa`, 29}.
It is incumbent upon whoever took anything from the school cafeteria unlawfully to give it back and the Mudrib must fear Allah and be honest with the authorizer so as to be cleared from liability before Almighty Allah. And Allah the Almighty knows best.
[1] Mudarabah is a partnership where one party provides the capital while the other provides labor and both share in the profits. The party providing the capital is called the rabb-ul-mal ("silent partner", "financier"), and the party providing labor is called the mudarib ("working partner"). In classical mudaraba, the financier provides 100% of the capital; cases where the capital is provided by both the financier and the working partner result in a joint mudarabah-musharakah contract. Profits generated are shared between the parties according to a pre-agreed ratio. If there is a loss, rabb-ul-mal will lose his capital, and the mudarib party will lose the time and effort invested in the project. The profit is usually shared 50%-50% or 60%-40% for rabb ul mal-mudarib.