My school has two full Maths Extension 2 classes and 5 (I think it may be 4 now) Maths Extension 1 classes. Only one of our Maths Extension 1 classes (the class I'm in) contains 26 students who all also do Extension 2 and are in the same Extension 2 class. No one in my class does only 3 unit Maths, they all do 4 unit Maths at the moment. The other Extension 2 class people are just dispersed everywhere in different Extension 1 classes according to timetable lines. There are a lot of people doing Maths Extension 2 at the moment and my Extension 1 teacher said she "hopes" that some will drop out by the end of the term to reduce large class sizes. Hence, for this term we have to do the 2 unit assessment tasks as well as the Extension 1 and Extension 2 assessment tasks. This accounts for those who wish to drop out of the course by the end of the term so that they have a supported 2 unit assessment task. That applies only for this term and then by next year no one would normally be allowed drop out.
According to my Extension 2 teacher, people normally would drop out because of the workload, not the difficulty of the course. Spending only 4 hours classtime per fortnight on the course means we have to go through all the theory in class and do the exercises at home because there isn't much time.