To dream about high school refers to the bonds and friendships that you made while you were in high school. What spiritual lessons have you learned? The dream may also 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.