The SAT/ACT test what students learn in school. Therefore, the more students goes to school, the higher the scores will be. The tendency to start prep for the tests too early in the summer before 11th grade is a common one, but ultimately foolish. The time would be better spent on any number of other activities that will make for a stronger application and ultimately better personal grown. Students will learn much more about the world and themselves scooping ice cream than taking a stack of practice tests over the summer.
There is a fair amount of math content on both tests that isn’t normally covered until the first half of 11th grade such as complex numbers, basic trigonometry, circle equations, and absolute value. It is much better to wait until these concepts are covered normally in school than trying to read ahead of normal course work. There are of course the really cool kids who are in honors math and go to math camp, but such students will probably excel in the math portion if they take it in August, and score even higher if they wait till December or March with no additional effort. Similarly, reading and writing/grammar skills will improve over time and scores tend to float up naturally with normal school work. Students should try to read as much as possible – anything without too many pictures is better than nothing.
The danger is students rushing into test prep too early, taking the test early in 11th grade, scoring ok but not great, then feeling the need to prepare again for December, rinse, and repeat in a Sisyphean quest for a few more points that won’t make a difference for admissions. Better to hold off until the second half of 11th grade or the December test at the very earliest. Prepare once, do well, and then worry about making varsity volleyball or getting the lead in the school play. You can take the test again if needed, but best to go in with a positive attitude.