Is taking a kilogram of sweets designated for a school party without the knowledge of the administration or the students considered haram (forbidden)? And what must be done to atone for this sin?

The questioner and her friend are agents for the school in preparing the ceremony, and an agent acts within the limits of his agency. Therefore, their taking a portion of the sweets before distributing them is incorrect, and they must return what they took or its value to the administration, or seek permission from it.