How to write an apology letter when i didn't do anything?

The majority of my cl was misbehaving when we had a sub the past 2 days and so now we all have to write a 2 paragraph apology letter: 1 to her and 1 to the sub. I don't want to fess up to something I didn't do; i was doing my math study guide while most of the cl was screaming and dancing around the clroom and the sub was trying to get the movie to play. My teacher says that if a couple people were not misbehaving, the sub would have given her those names so shes punishing everyone. We aren't allowed to say that weren't misbehaving and we have to turn it in monday or we get detention. How do I write it?