What is one of the opportunity costs of attending college?

One example of opportunity cost of attending college is "the highest valued alternative one forfeits to attend college".

So basically, the opportunity cost would essentially be anything you would have done, and the money earned and saved had you not attended college. When people go to college, they come to a decision to put off full-time employment or a career for four years. Depending on one's scenario, the opportunity cost could be higher.