It was a novel classified as young adult.
I don't remember the character names.
First off, it was a full-length novel (around 300-400 pages, but I might be off by a lot). The cover is a sunflower-yellow background with a large white flower put on the solid yellow background.
In this, there are students who are under strict dystopian rules. This is the south side, and the north side is free. The protagonist is a girl who rejects society, while her younger brother accepts it. The main character finds refuge from society in art class, where the teacher is lax & does not follow the dystopian rule. One day, the art teacher went away, and a person named Mr. Green…...... (I forgot the rest) said the art teacher got suspended for not following the rules, and he was the new teacher of the new class of practicing rules of that society.
The first thing he did was take away everyone's jewelry. Then everyone's head was shaved by a machine, and I specifically remember the girl remembering her head was raw. She misbehaved some more (I forgot what), and she was deemed to become a stan (or stak? Ston? Idk, something around that). They took her to get a chip implanted so drones could track her anywhere, and she was loaded into the truck for the operation to remove her brain. Then, Mr. Green... rescued her from the operation before the truck left for the surgery center, dug out the drone chip from her flesh to prevent drones tracking her and escaped to the north side for freedom.
Another thing I remember is that on the school bus, the kids talked with their mouths closed, since talking in school was banned.
This had to be before 2017, since that is when I read it, but it could be MUCH older (just got it from library).