8th grade teacher. It's really easy to see when a student is playing games. If I am in doubt, I don't take the computer. But if I know the kid is playing games, I do the same thing-- approach, close screen, take the (school-given) computer away. That said, I email the parents immediately AND I email my colleagues and say, "Jimmy was playing video games in my class and I have confiscated his computer. If he needs it for your class, please let me know and I will deliver it."
Also, of course the child (any child, the vague child, not OP's child) said they were doing homework. If it was MY kid, I would not believe him.
I love that you try not to use computers! I think moving all kinds of textbooks, writing, and everything to screens is changing the way kids learn how to think, read, and process information.
I think it's awesome that they're moving textbooks to online -- so much better for the student's physical health instead of carrying around all those huge books.
I know DS' school, they use Chromebooks, but they also still write in notebooks, do math on paper, etc.
DS/my school district uses (school given) Chromebooks for all textbooks in high school. I'd love if instead they had physical books in the classrooms that stayed in the classroom for reading & discussing in class. Textbook online so kids aren't carting home multiple heavy books, yeah that's okay.
I also know that more kids are getting glasses earlier due to the increase in screen time overall. How about we use less of the screens during the 7 hours of schooling.