My classmate stole one of my files. What can I do?

I attend high school in Slovenia. We are currently making an HTML project on a group server and my classmate has stolen one of my CSS files. He just copied it to his directory, but I cannot delete the file because I don't have the owner's permissions. What can I do? Is there any other way to circumvent the owner's rights or do I have to tell our teacher?

I know that it sounds weird, but I'm really feeling hopeless.

Thank you for your attention.

Sorry, but this is not a WinSCP question. In general, you indeed cannot delete files of other users.

Thank you, I appreciate that you took the time to answer this question. I will solve the problem by myself.
Kind regards, Matija