what does it mean if a girl is coed? does it mean she is a transexual?

I've never heard that term used in the hobbying world,but since it usually means both males and females,I'm going to assume that a provider used it in her ad and that it means that she will have sex with men,women and couples.

Or it may simply mean that she's a student.

... the only time I've seen the word coed used in the hobby is along the lines of "Sexy Coed Suzy Available for Action".

Coed, comes from the term coeducational, which described colleges and universities which accepted both men and women 60-80 years ago.

By saying she is a coed, she is implying that she is young, a college student, and hoping to finance her higher education through the hobby.

a 29 yr-old provider who hasn't stepped foot in a classroom since high school and advertises as a 'coed' so she can appear to be much younger. Also, as a justification for higher donation rates because she is better-educated and classier than the average provider.  
Before I get flamed, know I'm kidding (I kid because I care) to some degree.  But there is an element of truth  in what I say.