As you said, your fast food job sucks. An aide job will most likely be better, and even if it isn't, it can't be any worse- you'll at least be making more money. Might as well go for it. All you have to lose is $900, which I sure as h*ll wouldn't want to spend for nothing, so I'd look into financial assistance for the class. Contact the dept of labor or ask the school about scholarships
You can probably stomach a lot more of the dirty work than you think. We all can. And you don't have to worry about not being strong enough- most CNAs, including myself, are small young women. We do get more muscle tone by working, but it's not the same as "working out." Personally if I'm doing anything that requires muscle I throw my whole body into it, not just my arms.
The only thing I'd be concerned about is your weight. What is your activity level now? Working as an aide, you spend 8 hours walking around, usually at a pretty good clip. You probably don't move around that much- I know I didn't. Nobody has time to run around 8 hours a day unless it's at a job. So you'd probably lose a few pounds doing that, but will you have trouble walking all day? Bending/crouching down a lot? Scurrying from room to room? Pulling people up in bed and rolling them over? Stepping over tons of crap in tiny rooms jammed with equipment? If you think you can handle 2 or 3 weeks of that you'll probably get used to it and be fine after that. It's that initial adjustment that you have to worry about.