Complex sentence: When originally opened it only included grades 6-7 and grades 9-10 and each year continued to drop the middle school grades, until it became a total high school, from grades 9-12 in the 2003-2004 school year.
Split into simpler sentences: When originally opened it only included grades 6-7 and grades 9-10. They became a total high school from grades 9-12 in the 2003-2004 school year.