二年级 (èr niánjí) is a noun meaning "second grade" or "second year." It refers to the second year of study at any educational level, and follows the pattern of number + 年级 used for all school grades in Chinese.
我女儿今年上二年级了。
Wǒ nǚ'ér jīnnián shàng èr niánjí le.
My daughter is in second grade this year.
二年级的课程比一年级难了不少。
Èr niánjí de kèchéng bǐ yī niánjí nán le bùshǎo.
The second-grade curriculum is quite a bit harder than first grade.
他是二年级的语文老师。
Tā shì èr niánjí de yǔwén lǎoshī.
He is a second-grade Chinese language teacher.