Should I be required to pay 300 for being thirty minutes late?

I’m a high school student, and we had a group practice scheduled at 11:30 AM. The night before, some members of the group were chatting in our group chat, saying that anyone who comes late would have to pay 10 for every minute they’re late. I ended up arriving 30 minutes late, and now they’re charging me 300.

I’m unsure if I should pay it, because: 1. It was just being discussed in the group chat, I thought it was a joke, and not everyone agreed to it. 2. They said the majority agreed, but I didn’t see any official vote or poll about it. 3. 300 feels like way too much for a late fee, especially for students like us.

I’d really appreciate your kind advice on what to do.