You should register for High School the year before you start attending. For example, to start high school in September 2011, you register in December 2010.
If you are already attending an Elementary or Middle school, you will be able to register through your current school for High School for the next year. If not, you must register directly with the High School you wish to attend. If you register late, you need to speak to the school principal, and may not be able to attend the school you want to.