Wrong for not working unpaid overtime?

Would I be wrong if I tell my supervisor that I’m not working next Saturday?

I work at an elementary school as a Liaison. There are 10 of us, and we’re each assigned to a school to work at. My supervisor is a lead liaison. I only make $30k. I work at a Title 1 school, so we do a lot of family engagement events after school at least once a month for about 2-2 1/2 hours. I don’t get paid extra. At all. Well, my school is having another event next Thursday in the evening. I’m not getting paid extra. Okay, fine. Well, our family engagement team just sent the liaisons and our supervisors an email stating that they are having an event next Saturday from 8:00am-12:30pm. Again, I’m not getting paid for that. They specifically asked me and another liaison to attend because they will “drive by” our schools. The event has nothing to do with my school or the families who attend that school. It’s something they planned for a community event.

I’m already working late next Thursday for my school event, which I’m happy to do because it’s for the students and the families. My supervisor wants me to participate in an event that the team is having that has nothing to do with my school next Saturday. So, that’s two days next week we’re I’ll be working overtime, but not getting paid. I’m honestly thinking about telling them I can’t do the Saturday event.

Would it be wrong? Does it sound selfish? I work so much overtime during the week and not get paid for it. I don’t want to work Saturday morning, especially after staying late that Thursday. Honestly, if I would have known I had to work overtime and not get paid, I wouldn’t have taken the job last year. I was told in the interview that we would be compensated for extra time worked. I haven’t been yet and I can’t even tell you how many hours of overtime I worked.

Am I wrong?

Edit to add: Thank you everyone! We have tried asking for extra money, but it hasn’t happened yet.

I emailed my supervisor and told her that I will not be attending the event. I’m waiting on a response….