An interesting one - and my day job for three days/week: teaching teacher how to run a class. And every now and then, I assist training companies with development of sales trainings etc.
First, make sure you have an agenda/time schedule for the hours/day. On top of that: mention the agenda of the session and tell people what YOU expect and what THEY can expect. And... stick to it. Really, do it. Be very precise with time management. Make sure you divide various sections of the agenda in reasonable parts and achievable periods: '...it is possible to do this in 15 mins'.
Use a time timer (a backwards clock). Show the time running towards 0 mins. Finish is it off with something funny: exploding dynamite, an alarm clock, a funny YT vid / gif. It works. Be clear. Change the action: after 15 mins working, get them in motion: put the results on a large paper and hang them on a line using cloths pegs (or how these are called). The difference in 'modus operandi' pulls attention and people will pay attention. Make sure you have a break every 45 mins. (by colleagues referred as the 'Menno Pause' - instead of menopause)
Finally: run the first slide again: the one with the session's goals. Discuss the results (what was the goal and did we reach this goal), including the opinion of the participants and discuss the process (how did we do?). It works. I did a lot of training for Royal Dutch Customs, Fire brigade officers and civil servants of the income tax department, including their forensic department - tough bunch to get their attention and trust...
A few weeks back, I helped a company to set up a sales training. We booked a hotel + conference room in a remote area of The Netherlands (yes we have remote spots). The sales training was set to start at 08:30 Monday morning. Guess what: everybody entering the room after 8:30 was commented as being too late. And no, we didn't accept an apology. We sent them out, telling them: 'Lesson one! Be on time when you visit a new client/customer. You are too late; I will not do business with you!' And contacted their boss about the situation (all bosses knew that this would be our opus moderandi). We had a quiet bunch of those that stayed that day...
Menno