You can delete a student via the Teachers Dashboard. This will disassociate the student from your class and school. Please remember that you will 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.