I would segregate beginner, intermediate and advanced trainees, and would require them to go through some sort of training/assessment to proceed to the next level.
For example, I might have a very simple/basic exercise selection, with loads/WODs unlikely to injure anyone, for beginners, and would require them to demonstrate proficiency in the movements before advancing to intermediate. For the ones who just can't get form down I would offer the option of personal instruction and would allow them to stay in the beginner groups, but not allow them to advance.
For intermediate trainees, their WODs, I might replace "hard exercises" for easier ones (for example, if a person can't clean properly, I'd have them deadlift, or some shit like that) until they can show they can perform the exercise safely. They would be required to showcase a fair degree of proficiency before proceeding to advanced (again, private instruction, or maybe small group training in things like oly lifting could be options).
Advanced would be people who are fairly proficient in the technique of pretty much all the lifts.
Basically, I would work in segregation, scaling, personalization and basic requirements for each level.