There are a couple of resources I would like to share about. The first one would be Kahoot. I have seen our math teacher use this resource. Just like was described, he uses it for review and when class times have been adjusted. This keeps the kids very engaged. I have not yet seen him use it in a substitute situation. I have also heard of some of our teachers using Quizlet which is another resource similar to Kahoot.
The other resource I have seen used is Remind. I work with the head volleyball coach and she has used Remind as a communication tool with her players and their parents for several years.