What if you fail?

[Nismo]

I've been interested in joining SF for some time now. The only problem that I have is that I am a U.S. Resident and as you all know that you have to be a U.S. Citizen in order to turn in a packet.
Well I went ahead and joined the Army to speed up my citizenship status. It's now just a matter of time till I get my citizenship. My question is this.
What happens if you pass through selection but fail out of the school process? My friend that was part of 10th group (as support, he was not SF) told me that if you fail then your MOS changes to 11B. I thought that if you do not make it then you just get sent back to your unit. I do not doubt my ability to make it whether it is physically not mentally, but there is always that small chance. If for some un-Godly reason I were not to be able to make the cut then I wouldn't want my MOS to be changed to 11B.
If this question has been asked before then I sincerely apologize, I did use the search engine but nothing appeared. Well thank you in advance and I hope one day I experience what some of you have experienced.