It depends. If you rock out before you complete A school, you will go to the fleet as an unrated E-3. Once in the fleet you can choose to "strike" for any rate within your department. Basically study and try to get some OJT and then when you finally get promoted to E-4, you will have a rating. This is basically true of any rating in the Navy...if you fail A school this is what will happen. It just so happens that Nuc Fld A school is harder than most.
If you rock out during NPS or Prototype, like I did, you will already have a rating. Upon completion of A school, I was an E-4 Electrician's Mate. When I failed NPS, I was sent to the fleet on a non-Nuke ship and served the remainder of my enlistment there as an electrician.