I love the idea of using time. I used to do that a lot when I was coaching high school athletes as well. It automatically kept everyone together. For example, if I did an aerobic set it might be:
3x200 descend (1-3) without changing stroke count at 30 seconds rest. (I can count to 3) Then I would mix it up with a variety of focal points or specific challenges. But some kids would average leaving on 2:30 and other kids would leave on the 4:00. But all of them spent 30 minutes with their heart in the same zone, working on the same things. So the workout fit everyone. I don't have to count yards or worry about kids arguing about who miss counted or cheated because they did one less.
My master's coach does the same thing. It makes practices much easier to manage.