Can someone explain how music education work in America?

In my country, we generally take graded examinations from ABRSM or Trinity. In US, it seems that there graded exams are not common. How are students graded? I'm planning to minor in music in a American college, and am required to participate in ensembles/orchestras/choir/etc and take practical classes. Can someone tell me how are students graded in these practical classes and ensemble participations? And for the ensembles, audition is required, but how do piano students typically fit in? Because there seems to be a lot of piano majors/minors but little pianists required in the ensembles and choirs?