When should your student sign up to take the ACT or SAT? The answer depends on what grade they are in, and what the deadlines are for the schools they want to apply to. But the short answer is: earlier than you think! That's because more and more schools are offering priority deadlines, which means your student will likely be applying to schools with November (even October!) deadlines if they want priority access to things like merit aid and scholarships.
In an ideal world, students will have time to prepare for each exam, utilizing previous scores to inform test prep strategies. Having test scores is also the only way you can make a solid college list, as determining your best-fit schools will include an evaluation of your test scores. Every student's plan will be different but, in general, a wise approach is to take one exam in the fall of junior year, with a second exam in the spring. For juniors who have missed the fall test dates, signing up for an early spring exam (the deadlines are listed below!) should be a priority, so that you will have scores in time to prepare for a second June exam (if necessary). This will still give you time to firm up your college list before summer essay and application writing begins.