It's some of both. Parents do text their kids during the day, but kids also pull out all the excuses when caught.

Even when I was in high school "I was responding to my mom" was the go-to excuse when caught using a cell phone. I had one teacher who would actually read what was on the screen (this was before locking your phone was common and probably lawsuit material today, but things were different) and call kids out when they were lying. The threat of having a teacher read your text messages was enough to put an end to cell phone usage in class.