noun
head; chief; chairman; president
General term for the top person in an organization, committee, or government. 主席 is the standard kanji for this meaning, especially for political leaders (e.g., Chinese President).
彼は党の主席に選ばれた。
He was elected chairman of the party.
中国の国家主席が来日した。
The President of China visited Japan.
noun
top seat; first desk (in orchestra)
Refers to the principal seat in an orchestra section (e.g., first violin) or the highest-ranking position in a seating order. Can be used with either kanji form, but 主席 is more common for this sense.
彼女はオーケストラの主席バイオリニストだ。
She is the first violinist (concertmaster) of the orchestra.
noun
top student; head of the class
Only when written 首席
Specifically refers to the highest-ranking student in a class or school. Written as 首席.
彼は首席で大学を卒業した。
He graduated at the top of his class from university.