Use a timer - Set a timer out in the break area. This gives everyone a clear signal that the break is over. If possible, have the refreshments nearby. This makes it easier to stay close to the room. Be sure to have signs posted if food and drink are not allowed at the workstations.
Computer training is stressful and physically demanding for both the trainer and the participants.
Use this time to stretch and teach exercises designed for computer users. Take 1 longer break for phone calls - Plan for a phone call break, and announce it in advance. If you typically take two fifteen minute breaks twice a day, change it to one 10 minute break and one 20 minute break. Let the class vote when to schedule the longer break. Time breaks during exercises - Independent practice exercises are an important part of every computer class to determine if the skills are being mastered.