My son never took a course. He just had a couple prep books and did timed practice test sections, then reviewed his answers after each section. Before two of his ACT exams he studied hard, and the other two he didn't study quite as diligently due to time constraints. He improved up to six points on some sections, and at least a couple on every section.
Keep in mind, if a kid takes the test multiple times, that is over the course of a year or two. They learn more in school during that period, and then also get more comfortable with the ACT format each time they take it. And just improving one or two sections per test makes a big difference. At USNA they only have to focus on math and English.