How can a coach get his football team to perform better if he suspects they are exhibiting social loafing?

The coach could grade their performance.

Social loafing refers to the fact that a person might not produce much effort when in a group, as opposed when alone. So, these kids know that together they are pretty good, so they don't have to try hard; but when they are alone, they do. In order to prevent this, the coach might want to grade their performance, because then they will try harder.