I hate formal ski offs. Tux rentals are soooo expensive.
There are 3 big factors for lesson splits
Ability breaks down between kind of turns being made and steepness. There are lots of questions you can ask to figure out what kinds of turns are being made (do you use pole touch, make big turns vs little turns, stay in the fall line vs go across the fall line, what's the first move you make to start a turn, etc.). For steepness, it helps if you can see a variety of trails from the meeting area and just point until you find what the guest is skiing comfortably. How the guest answers the questions is almost as important as the actual answers because that's how you know when they don't know or when they're bluffing. I can't give you a magic formula for how to know these things, but if you've taught enough lessons then you'll know.
Speed/pace is a mitigating factor. Slower speeds moves people down a group for the ability they have. Faster speeds move them up.
We try to separate out kids from adults when we can. This can sometimes make the teaching effort easier better than closer abilities or speed.
The bottom line is that you have to be prepared to teach multiple lessons in one lesson. This can mean giving completely different tasks to students or just variations. Another option for no ski offs is lesson coordination. If you can keep similar level groups close together, you may have an option to move students between groups.