Can't leave a plan

Hi, I am new to Planner. I am a university student with Microsoft Planner, and I have been added to a plan shared with a group named "EOP Anti-Spam Inbound Junk Email Folder." I can't find the option anywhere to leave the plan, and the group doesn't appear in my Outlook group tabs. The plan also shows that there are over 16,000 group members. Could this somehow be tied to my organization?

I don't want this plan to clog up my "My Plans" page, but I can't find anything to help me know how to leave!

Thanks