Buzz,
As someone who graduated from Sheffield almost two years ago now (started April 2017, same class as you), try to get the ADX out of the way BEFORE CLASS if at all possible. Then, you don't have to worry about it at all; that alone is worth the extra $150 you'll need to pay to take the exam. If you cannot get it out of the way prior to class, be ready to take it when you get there. That is to say get all the way through whatever materials you're using to prepare for the ADX BEFORE you arrive @ Sheffield. Then, you'll only need to do some light review every day, allowing you to focus on your studies. When I attended Sheffield, they started offering the ADX late in week 2, IIRC. Get the ADX out of the way either late week 2 or early week 3 at the very latest. By this time, class will be getting MORE intense than it was for the first two weeks. Sooner you get the ADX monkey off your back, the easier and better school will be. Trust me when I tell you this, please!
Oh, in addition to the pdf files they give you access to when you sign up, be sure to take advantage of Sheffield's ADX PowerPoints; they're great! They walk you step-by-step through the performance charts and graphs. Be sure you go through the one on the HSI. The HSI is the horizontal situation indicator; it's a cockpit instrument like an old VOR and RMI combined. You'll NEVER see it again after the ADX test; you'll never see it on the job, either. That said, you still need to learn it, so you can get those questions right; I promise you that you WILL see HSI questions on the ADX! Sheffield's PowerPoint on the HSI is excellent, and I'd strongly urge you to go through it before you take the exam.
Before I went, I went through ALL of Sheffield's pdf files they use for ADX prep. I spent four hours a day for two months going through them. I went through their PowerPoints too. I got all the way through them before I arrived. When I got to Sheffield, I spent about an hour a day reviewing all the files, because I was rusty on what I'd studied early on. I was ready to take the test the first day of week 3. I think it was on a Sunday, so I had most of that weekend to do a final, marathon review session; I had Friday night and all day Saturday to make sure I was ready to take the ADX. I got a 91. Mission accomplished.
Good luck to you! I can't believe it's coming up on two years since I attended Sheffield. Time flies! Anyway, try to take the ADX before you arrive; it's well worth the extra money. If you can't do that for whatever reason, then be READY to take the exam when you get there. Then, all you'll need to do is light review every day, so you can focus on your studies. Sheffield started offering the ADX in late week 2 or early week 3; take it by early week 3, at the latest. Do those things, and you'll pass the ADX. Your time at Sheffield won't be quite as intense, and you'll have a much better shot at graduating. If you have any questions between now and graduation, don't hesitate to PM me. I've literally been there, and done that; I've walked in your shoes successfully. Best of luck!