。
I ended up getting expelled from high school.
Literal
I [topic-は] high-school [object-を] dropped-out-became.
The pattern ~になる with 退学 gives a slightly different nuance from 退学する: 退学になる implies becoming the one who got expelled (passive-like outcome), while 退学する is the more neutral 'drop out.' ~になる with passive/result nuance often suggests it wasn't entirely the subject's choice — a quietly resigned framing of an unwanted outcome.