what could managers do If some key person get out of the job ?

You know every job must has some key staffs.They are very important for the work so for example in a business of course the managers are important but secretaries and accountant or the financial managers... So many people are playing a important role in a business. So let's just make an example around us .

Our school- EF International languages school .
Some for a school the key staffs should be teachers and someone who are working for each parts.But If some of the key staffs are get out of EF .So what can we do ?

So I think there are several things we could do about to solve this problem :

• Engage someone to replace them as soon as possible.
• To guard against this happened we should not give one person many important things to do we should divide a work into several parts so if that happened we won't lost to much
• Maybe train other staffs to do their jobs

So what can I think out just those three things but I know there should have more .But now my brain is not work cause is too late now... so I am gonna sleep now.... ZZZ~~