major league(s)
An area, echelon, or sphere of great competition, success, power, achievement, etc. Refers to the major leagues of sports teams. I know you're new here, but you need to perform much better if you want to stay in this law firm. You're in the major leagues now. Welcome to the major league of politics, senator.
major in (something)
To complete a college or university degree with a particular subject as its focus. Originally I thought I would major in English literature, but I switch to psychology in my second year. Samantha's majoring in economics, with a minor in history.