You can delete a student via the Teacher Dashboard. This will disassociate the student from your class and school. Please remember that you may not be able to create a new account with the same username that has just been deleted.
NOTE: You cannot delete students when using Google Classroom - you should do this via their interface.
Alternatively, you also have the option to Archive a class of students, which can be useful at the end of a year when students are no longer at the school.
Users also have the ability to delete their accounts in the app. See instructions here.