The highest math on the test is Algebra II. So after students have taken that class in school they are ready to take the SAT. Some students take Algebra II in 9th grade or even in 8th grade. Are they ready at that time? For the math section, the answer is yes. Nothing they learn in high school math after Algebra II is likely to help them on the SAT and the further they are from their Geometry class, the more likely they are to forget some important geometry facts. As far as the English section, however, it is likely that they will improve as they get older, particularly if they read a lot and study vocabulary. Most students do wait until their junior year to take the SAT but it is fine to take it earlier.
The SAT is offered seven times a year. On three of those dates the College Board has an option called the Question and Answer Service, whereby students can choose to get the questions from their test mailed to them (for a fee). If they are intending to take the test again, I strongly recommend choosing this option when they sign op for the test. It is available for the tests given in October, January and May.
I have often reviewed a student’s test with them after they’ve received it in the mail. They are usually encouraged when they see questions that they missed due to carelessness or other reasons that could be easily remedied.