When people attend high school class reunions, they often compare their own personal and professional successes and failures to those of their former classmates. this means that classmates are a (n) : ?

Given the aforementioned context, classmates may be considered a reference group.

Explanation:

From a very young age, people learn to guide their behavior, actions, and to evaluate its outcomes by choosing ideal figures as examples of proper conduct. Often, these are our parents and may include our peers at school, to whom we naturally compare our personal attributes and abilities. Reference groups are an individual's "ideal figure", to whom their qualities, attitudes, values, behaviors and conquests, are compared.

When people attend high school class reunions, they often compare their own personal and professional successes and failures to those of their former classmates. this means that classmates are a (n):

Reference Group