Anthony strongly recommends not waiting to begin your SAT or ACT prep.
Once you have finished your freshman year, it’s time to get started. We have a full, free guide on the many reasons behind this philosophy here:
The short version: the material your child is yet to learn in school is very limited re: its application to these tests, and can be picked up in a few hours of study. Furthermore, much of the material tested by the SAT and ACT is taught in grade school and middle school, so each day you wait is another day that much of the relevant material slips from the memory! It’s far wiser to begin as soon as possible, pick up any missing material elements, and tackle this process as actively as possible.