'I finished school' is a correct and usable sentence in written English. You can use it when referring to completing your primary or secondary education at a school. For example: "I finished school five years ago and now I'm studying at university."
I finished school, but who wants me?
"I was lucky when I finished school at 16.
They looked after me while I finished school.
When I finished school, N.E.C.C. helped me get a job.
I finished school in Lithuania before moving to the UK.
I finished school while he worked a year at a veterinary clinic; then I got pregnant.
Alan: Shortly after I finished school I got a reporting job at The Hartford Courant.
"I finished school at 4pm and by 5pm I was working.
"When I finished school, everyone wanted to go to a good university and become a lawyer or a doctor.
At some point, when I finished school in Zurich, I suddenly realised that I was nobody.
When I finished school, in June of 1937, I was twelve and a half.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK