My son had a similar experience he went to a preforming arts high school when we lived in Phoenix then junior year we moved to the middle of nowhere in Iowa where one school serves all grades for all the surrounding teeny tiny communities. He was ahead of the game when we got here and had to repeat a lot of the same classes which bored him and his grade spiraled.
Our first teachers conference one of his teachers praised he is a brilliant kid because he’s always raising his hand in class and knows all the answers but had yet to turn in a single assignment. My response was that he’s taken the class before so he’s not actually learning anything he didn’t know before.
He does read a lot and thinks out of the box and finds ways to learn new things outside of school but he graduated a C average student and he owned it because he was uninterested and gave no effort. He did the bare minimum “requirement” to squeak through.