I'm not sure you need to take pre reqs before. If you need to take them for a program, OK. But I wouldn't hold off taking the test until you finish a specific class if that's what you mean. In a perfect world, you'd remember everything you learned in middle and highschool. Although the TEAS is hard, it's really just basic concepts. I wouldn't overwhelm myself studying advanced biology and chemistry books or complicated math.
The ATI book is really useful. I think sitting down for a month or so and going over everything is your best bet. If that comes before you take pre reqs, I think you'll still be fine.