Use United States as the proper noun, U.S. (with periods) as the adjective. We usually discourage use of U.S. as a noun, and we don’t capitalize the article the in front of United States in running text. Typically, we use U.S.A. only in addresses.
He’s back in the United States.
not: He’s back in the U.S.
Lowercase the word, even when referring to IU, unless the word is used as part of the full name of an institution.
Indiana University, the university
See also Indiana University for official IU campus names
Avoid. The term means juniors and seniors only; it does not include sophomores. Do not use the elitist-sounding phrases upperclass students, upper-class students, or upper-class men. To refer to nonfreshman undergraduates, use sophomores, juniors, and seniors instead.