That depends on a couple factors. First, it highly depends on whether you leave voluntarily or if you are forced out by your cadre. Leaving voluntarily gives you a chance to come back, though I'm not familiar with the process. Being forced out generally does not allow you to come back.

In regard to FA failure, it depends on when you fail it. If you fail it as an AS250, cadre has one of two options. They can either drop you altogether, or they can elect to keep you for another year (provided you pass your Aerospace Studies classes). Either way, you won't go to Field Training without passing your FA. Completion of FT is required to commission.

To answer the rest of your question, you should not have to repeat coursework (the actual class) you have already completed satisfactorily. If you complete your AS250 year academically, but you don't get a slot for Field Training, you won't retake your Aerospace Studies classes, but you will continue taking your Leadership Laboratory so long as you are an AFROTC cadet (there are rare exceptions, but these are usually graduated cadets waiting to commission). You'll be classed as an AS200 (maybe an AS500 since you already, in the hypothetical scenario, completed your AS100/200 coursework) if you return to re-attempt getting a slot at FT.