I took mine in August last year, right before senior year--just before they got rid of the analytical MC questions. My applications weren't due until December and January, so it gave me enough time to retake it if I needed to (I didn't). I would suggest taking it when you aren't taking classes, in order to focus on it entirely. Because I hadn't taken geometry since 8th grade, I needed some time to review. Plus, I was insanely busy first semester, and I don't think I would have done as well. I studied from June to August during the summer, stopping entirely the week before the test.
If you're very very concerned about the math section, both Princeton Review and Kaplan offer online reviews just for math, at a fraction ($20-100) of what it would cost to take a full course in person (last time I checked, the least expensive ones were $400). Since the test is on computer, this might work. I'm not even sure if you can still take the paper GRE in the United States--they are trying to phase it out completely.
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