To dream about high school may be telling you that you need to start preparing for the real world.
To dream that you have to repeat high school, suggests that you are doubting your accomplishments and the goals that you have already completed. You feel that you may not be measuring up to the expectation of others. The dream may occur because some recent situation may have awakened old anxieties and insecurities.
I'm not totally attached to dream interpretation, but I'm just saying if I were, this would be, like spot on.