彼女はクラスメートみんなと仲良くしている。

English Translation

She is in harmony with all her classmates.

Classmates in English can be either

But please be noticed that 同級生 and クラスメイト don’t always mean the same in Japanese.
同級生 means “fellow students in the same class of my school”. クラスメイト in Japanese means “fellow students in the same cluster and the same class of my school”. A class is usually divided into several clusters, and a teacher is assigned to a particular cluster in Japan. Students in a different cluster but the same class don’t learn together in the same classroom.