。
She's my classmate.
Literal
She [topic-は] my classmate is.
クラスメイト (or クラスメート) is the loanword for 'classmate,' fully standard in Japanese alongside the older Sino-Japanese 同級生 (literally 'same-grade-student'). Both are widespread; younger speakers and casual contexts skew toward the loanword. The polite copula です rather than だ keeps the introduction polite — typical for talking about someone in front of others.