If you enter the name of your high school and don't see it in the returned list of schools, it may be because there are too many with similar names and not all are appearing in your view. If you include quotes around your high school name and then search, you will see that appear first and can select it. The list is updated periodically through The College Board CEEB code license.
If you are not able to find your high school, you can enter it manually by clicking on "I cannot find my school."