AITA for taking away my step daughter's summer

I have been in my step kids life since they were 3 and 4 years old. I love them like my own and have helped my husband raise them. Their sees them every other weekend but she's never been the one to discipline them. She prefers to be the fun parent and leaves the disciplining to us which sometimes causes problems when we clash.

The last week of school my step daughter who is 15 was in a fight with her best friend. I don't know what it was over. Just that it was stupid. She heard her friend talking about her in class and she through a large book at her which ended up hitting and damaging her Spanish teachers smart board. She got suspended for the rest of the school year. When we were in the principals office talking we were expecting a bill but we found out the teacher got billed for the smart board. I found that strange because I thought when a student damages something the parent gets billed. Both me and my husband talked and didn't think that was fair to the teacher so we wrote a check to the school for the smart board and in return we told my step daughter she would be getting a job for the summer to pay us back for her mistake. She told her mom and her mom got angry saying that's not fair to take her whole summer away when we chose to pay for the smart board. We chose to pay for it because it's not fair the teacher got billed for it when she is the one who broke it. Her mom said if they billed us she would understand but we made the decision to pay for it plus it was an accident. Hitting the smart board was an accident but she shouldn't have been throwing things.

The teacher shouldn't have to pay for the smart board. They already don't get paid enough. She needs to pay for what she caused. Their mom just doesn't agree with our decision but we made my step daughter get a job. She's been paying us back slowly which each pay check so far but I'm now wondering if we were wrong.