Some colleges require 2 or 3 SAT II subject tests.  I understand that these tests are something that high school students learn in their classes as well.

With the ACT, students have the option of not reporting the test results to colleges, but all SAT I scores are reported to colleges.  So, if your student takes SAT I insted of ACT, make sure that your student is prepared for the test.  I have not looked into preparing for ACT, so I can not say much about it.  Preparing for SAT requires a lot of hard work.  The most basic item you student needs to have is the “Blue Book” published by the College Board, The Official Guide.  Have your student go through all the questions in the book.  Have them study again all the questions that they miss or do not understand.  If your student has hard time going through the book on his/her own, you should consider helping him yourself or consider hiring a tutor.  That Blue Book is that important.