Why is my classroom set to Spanish?

The Duolingo for Schools classroom should default to the language course of the majority of students in it. If nothing is set, it defaults to Spanish, as it is by far the most common language taught from English, but the teacher can reset the language by clicking on the classroom and clicking on the last option on the menu on the left, "Classroom settings". If you go there and follow these steps, it should change the language to the one you wish. You can also simply ask your students to join, and the classroom should automatically change to the language all of them are learning.