When students have left a school, you should put them into the 'students no longer in my school' section of the roster.
These students will remain there for 6 months, giving them time to join a new school or family account. You can also move them back into your main roster if they were removed by accident. If a removed student has not joined a new school or family within 6 months, their account will be permanently deleted.
If it is vital to have them permanently deleted straight away, get in contact with firstname.lastname@example.org