This is the 445th most frequent Japanese word.
卒業
Graduation.
Here, '卒業' is used as a noun indicating the act of graduating, and it is paired with the verb 'しました' which means 'did' to indicate the completion of graduation.
私は去年大学を卒業しました。
I graduated from university last year.
In this sentence, '卒業' is part of a descriptive phrase '高校卒業後' meaning 'after high school graduation'. It frames the context of the timing of the action.
彼は高校卒業後に留学しました。
He studied abroad after graduating from high school.
Here, '卒業' is used metaphorically to mean 'finishing or completing' something, in this case, the piano classes, beyond formal education contexts.