I believe it all depends on your individual school program because most all have different ways of doing things. In my school we are allowed to take nursing classes over the summer if they do not have a clinical/lab component. For instance first year students are able to take Nursing Fundamentals and Pharmacology during the summer.
My school is the same. A is for morning lectures, B is afternoon, and C is night/weekend lectures. If you choose group A you get to pick from 2-3 "A" clinical sites - same for B & C. They try to give each group a morning/night clinical option.