OK, here is how I look at the class vs prior history deal.
Class is a *what* skills/feats you are picking up; the name of the class doesn't have to indicate your career.
Prior History is *where* you are picking these skills up; this is either in military Service, a job/career, how you make a living, or at school.
You can be *in* a University and still *pick up* skills/feats that are unrelated to your degree...ie the Rogue in College debate.
Obviously you're not going for a Masters degree in Applied Theivery (unless it is a very liberal university on a very low Law Level world), but you might learn to Gamble (a class skill for a Rogue, but not for an Academic).
You could be in the Marines Prior History and you'll have to take 1 level of Marine class, to represent the basic common experience that all Marines share, but after that...
Want to be jar-head? take all class advances as a Marine...
Want to be a master tank mecahanic? Take some levels in the Professional class and concentrate on T/skills...
Character in the Marine's JAG? Take some levels in Professional concentrating on K/Law and P/Lawyer
Don't let yourself be limited by the "names" of a class or of a Prior History path (with exceptions of course...if you are in P.H. Marines, obviously you are in the Marines; University you are in some kind of learning arena). The "names" are just general descriptors for the most part, not "written-in-stone" mandates of what your character *must* be.
For example, a "Rogue" does not have to mean "thief"..a "rogue" could be a perfectly legit repo man,a locksmith, a long range amry scout good at skulking...you name it.
Well, that's my two-hundreths of a credit...
-Roger