Most of the time, progression from one level to the next has been dependent on the instructor's approval. When you're ready to move on, they'll let you know. There are typically prerequisite classes needed, but no exams or anything.
I'm in agility 2 and there are some more advanced people about ready to move on, and some beginners from my agility 1 class who will be here for another 3 or 4 rounds of classes, so the instructor does have to do some adjustment for skill level. Last class she broke us into two groups and had my group work on basic jump foundations while the other group was in the big ring. Then we switched and the other group worked a short sequence on their own while we went in the big ring. It's also a really big class (probably too big) so the ability range is fairly large.
I have done a few classes that are sort of an end in themselves, and the same people tend to come back over and over. For example, we have a "fun" obedience class which is basically a pre-competition level class. Some people in the class are quite advanced and like to mix things up, some have no interest in competing and take it for fun, and some (like me) aren't ready for a competition level class and take it to practice and improve the necessary skills. So there is a bit of a range in that class and the instructors modify it to the individuals. When Watson was having issues with group stays, we moved him across the ring to practice on his own, on leash, while other dogs were doing stays off leash with the owners 40ft away. When there are dog reactive dogs we don't do much off leash stuff, but when dogs are friendly we can do more like play musical chairs (dogs have to do an off leash stay while owners run to a seat). They often ask what people are interested in working on and if someone says "straight fronts" we'll do a lesson on that.
Generally if you can't make it to a class at your level you're SOL.