(i live in phx, az)
Most community colleges have open enrollment which means you do not need a high school diploma to take classes though they generally have a minimum age of 15 or 16 to attend.
Many homeschoolers take advantage of community colleges for their last two years of high school effectively getting dual credit for their work as both high school classes and college credit.
I believe the community college here offers GED prep courses.
But for specifics you would just need to talk to your local community college.