At my typical large public HS most people take the ACT in the school wide April administration. However in my group of friends, most of whom are looking at fairly selective colleges, most of us started doing testing in the Fall. I began in March of my sophomore year (mostly because I freaked out when I heard about the SAT format change), and was very glad to do so. I would really recommend that for a kid who takes Pre-Calc or even Algebra 2 as a sophomore, just because the math is fresh in their mind then. That allows you to identify weaknesses and have 2 summers, not just 1, to study intensively if needed.