Suspension
Use it for a leave of absence, a long illness, or a holiday period. You pick the dates when you suspend, so you do not have to remember to switch the account back on: it happens on the end date by itself. A suspended member cannot work inside the school but keeps everything they have done.
Removal
Use it when a teacher or student leaves the school permanently. They lose their membership in your school and their place in your classes. Their own Meta Typing Club account and their personal progress remain theirs.
Which one counts against the plan
Suspended teachers still occupy a place on the Free plan. If you need the seat for someone else, remove rather than suspend.