Is it permissible to sell the leftover biscuits from students at school that are about to expire, and deposit the money into the school's treasury, and would the one who bought them be sinful?

The biscuits that are in excess of the students' needs are the property of the donating entity. Therefore, one must refer back to them to ascertain how to dispose of them, either by returning them or by selling them and depositing their price into the school treasury. This is because taking someone else's money without their permission and consent is forbidden, as stated in His Almighty's saying: ﴿O you who have believed, do not consume one another’s properties wrongfully unless it is [property] by trade [in which there is] mutual consent among you﴾ [An-Nisa 4:29], and the Prophet, peace and blessings be upon him, said: «The property of a Muslim is not lawful except with his good consent». The school principal does not have the authority to dispose of the surplus, as she is an agent, and an agent's actions are restricted by permission.

If it is difficult to refer back to the donating entity or their response is delayed, and the principal fears the biscuits will expire, then it is permissible to sell them and keep their price until a decision is issued by the responsible entity. If it is also difficult to communicate with the responsible entity regarding the price, then it must be used for a general benefit of the school.