How to Withdraw a student
1) Go to the Student Record.
2) Edit their enrolment status:
3) Set the School year status as "Withdrawn": 
4) This will take them out of any classes they're currently enrolled in and will prevent them from being added to anything else moving forward.
5) Once you rollover to the next school year all withdrawn students will update to become "Past" students.
Additional information: Students currently enrolled as "This Year Only", will also update to "Past" students once you roll over to the next school year.