MoveIt is a free Chrome extension that aims to help you avoid sitting in front of your computer for too long. At intervals of your choosing MoveIt will prompt you to get up and complete a short exercise. You can set the intervals to be as frequent as every five minutes or as infrequent as every hour. You can also disable MoveIt altogether for the times when you absolutely cannot be interrupted. An overview of MoveIt is included in the video below.
Sworkit Kids is a free Android and iOS app that also offers prompts for short physical exercises your students can do in your classroom.