I don't understand all the hate this question is getting. I'm guessing the answers depend on the context a lot.
Context of my answer
I regularly hold company trainings. Adult education. These are full day IT trainings, that means 9 AM to 5 PM. In the contract we offer 90 minute blocks with 15 minute breaks between and a 45 minute lunch break.
Importance of breaks
A break is not a waste of time. Researchers disagree on the amount of time an average human can uphold focused attention for, but all agree it is measured in minutes. I have learnt 4-5 minutes at the university, and I have read some researchers claiming it to be up to 20 minutes. But you can't expect an average human to uphold focused attention for hours at an end. It is possible to renew attention, this is where the teacher comes in by changing the rhythm, the tone, the visualization, the exercise. But from time to time, breaks are needed. (The 8 second attention span you hear about is different, it is the burst attention span or whatever it is called.)
It is not just the attention though, but moving/stretching as well. Most workplaces have recommendations, and though there are slight variations, most revolve around the "work for 60 minutes, get up and move for 15". This is not only beneficiary to the health of the individual, but also (many studies point this out) improves work performance.
Answer to the original question
My actual answer is: explain to them why breaks are important, explain to them how getting up, stretching, moving can help them concentrate, but don't try to force them to do so. If sitting is the new smoking, then dealing with it is similar: you can't just forcefully take the cigarette out of their hands. They know their own bodies and what type of break refills them. And they will learn the importance of breaks the hard way.