What's the term for someone elevated in a position due to need?

A teacher might be teaching to only a few students, but then the number of students grows, so a new teacher is hired to help teach the larger number of students.

What would you call that teacher, or the action of hiring that teacher for that specific purpose (or any related situation)?

You could call the action as well as the teacher reinforcement.

reinforcement (noun)
1. the action of strengthening or encouraging something
2. something that strengthens or encourages something

This word is often used to describe the strengthening of a military force, but it can just as easily be applied to a more casual situation like this one.