How can I maintain performing supererogatory prayers despite time constraints due to evening school, given that I have started to abandon them except for the Fajr supererogatory prayer?

To maintain the regular sunnah prayers (Sunan al-Rawatib) and supererogatory prayers (Nawafil), read about their virtues and the reward for performing them. Seek Allah's help, strengthen your resolve, and hasten to perform them when their time enters. If the Dhuhr sunnah prayer is missed, it can be made up after its time. Do not let missing it be a reason to abandon the rest of the sunnah prayers, for performing some sunnah prayers is better than abandoning all of them.