Jealousy can be a big motive. For example, if one student is getting all the attention and good grades, another student might be driven to murder out of envy.
Then there's the motive of protecting a secret. Suppose a group of students are involved in something illegal or unethical like cheating on exams or stealing. If one of them threatens to expose the group, the others might kill to keep the secret safe. In a school setting, there are many complex relationships and emotions that can lead to such extreme actions.