There are only 50 in the game, and you need to do at least 40 of them as a requirement to unlock the Perfectionist achievement. Also, once a class is completed at Level 5, you are no longer truant the next time that class appears, allowing you to roam freely without authority figures trying to bust you. Additionally, the more classes you complete, the more abilities Jimmy will obtain. You need to have all classes completed at Level 5 as a requirement to unlock the Perfectionist achievement, and they do appear even once you finish them, so you may be required to wait longer than you need to in order to complete the game if you skip class. Your main focus should be completing the story line, with 2 exceptions: You have not had an errand show on your mini map for an extended period of time (keep in mind more errands appear the more you progress in the story line).
