2nde, or better 2^de, is a shortcut for seconde, like the English 2^nd. Here it means classe de seconde, the first class of French high school (similar to the US K10 grade).

In this sentence, de indicates belonging and 2^de (which is preferred to 2^nde according to the Académie) is the abbreviation of seconde, which is the first class of the lycée, which is the French equivalent of high school. In seconde, students are generally 15 to 16 years old.

So a good translation would be (assuming you’re American):

Jules: That’s Stéphanie. A new girl in 10th grade.