I quit the class because of “disagreements” with a new teacher on how to treat students and severe mismanagement of after school activities, and ended up causing issues long after I graduated.
To cut a long story short, the issues boiled over, and one day after class I told him I wasn’t coming back, and changed my schedule by the end of the day and served my admittedly-deserves detention. What I didn’t realize was that after I left, a lot of other people heard my side and agreed (or at least thought my response to him that day was funny), and either did the same, or stayed for the class credit and only did the bare minimum for a grade instead of what we all knew the group was capable of.
My “revenge” wasn’t exactly intended to have that effect; it was more so because I cared more about how he would treat the entire group when his beef was with me (passive-aggressively trying to make people dislike my presence by punishing them all doesn’t work great when these people have known me for years and wanted to do and say the same things), but it ended up being somewhat satisfying. A guy who was an asshole at the time ended up watching what should’ve been a great group dwindle into almost nothing within a few weeks because of his behavior, and having to work to rebuild the group itself and his reputation among the students (still working on the latter). Another plus, I started fiddling with some old tech and ended up where I am now.

I’m somewhat grateful for the things we both ended up dealing with as a result, and I hope we’re both better for it; people can change. Side note: I skipped school to go support them at an event almost 2 hours away, and seeing his face as he noticed me in the crowd was absolutely priceless.

TL;DR: butterfly effect taught jerk a lesson, and both of us grew from the experience.