In this story arc, Calvin stole her class notes and was trying to hold them hostage. Instead of meeting his demands, she waited quietly as a strategy. She was extremely angry.

Hobbes simply pointed out that, due to their hostage situation, they were able to stay up 30 minutes past when Rosalyn makes them go to bed.

Edit: in the arc, they literally hid behind a locked door until 7pm. They didn’t “act better,” hobbes is just pointing out they technically succeeded in staying up later