I was at school and one of my classmates stole one of my things
Dreams about theft can have different interpretations depending on the context and the way you felt during the dream. However, in general, a dream about someone stealing your belongings may suggest feelings of vulnerability and insecurity. It could indicate that you feel someone is taking advantage of you or that you lack control over a situation in your waking life.
Since the dream takes place in a school environment, it could relate to feeling disconnected or powerless in your academic life or school environment. The classmate who stole your item could represent someone or something in your waking life who is taking away your sense of control or leaving you feeling vulnerable.
Perhaps this dream is telling you that you need to be more assertive in your life and find ways to take back control of the situations that are important to you.