As
schoolgrrl noted (in a writeup since removed), "co-ed" has sexual connotations and is derived from the term
coeducational as it was applied to colleges when they stopped excluding female students.
It's generally inoffensive to use the term co-ed to describe a university, barracks, or other institution, even if its primary purpose is not educational in nature.
However, some people will be offended by the use of this term to describe a person. If you mean to say student or scholar or college student or something like that, say it. "Co-ed" not only sounds like something out of a letter to Penthouse Forum, it's also a completely unnecessary term that has the connotations of "studentess".
Of course, you can say whatever you want. Just know that referring to someone as a "co-ed" might offend even those not particularly interested in being politically correct.