Removing a student from a class there are two case scenarios.
1) Individual Teacher Accounts: Students can be removed from a class whether the student was added manually or imported with a file.
2) Teacher Accounts connected to schools/districts: Is a school/district is syncing data (students, classes with student rosters) option to remove will not be available.
Select the "Classes" tab.
Select the class in which the student you want to remove is located.
Check the box in front of the student you want to remove.
Select the "Remove" button that appears.
Select "Remove" from the confirmation screen.