To delete multiple student accounts follow these steps:
Click your name and then click School
Use the grade and class filters to narrow your list of students to the group you would like to delete
Use the select all check box at the top of the left column to select all of the students in the list.
Click Delete Accounts in the left column.
Click DELETE from the confirmation dialog
The selected students have now been deleted. You can restore these students and their data by using the Restore feature as described in How do I recover deleted student accounts?