According to the given statement, let's imagine that the teacher has x students.
He says that if as many students as he has now join his school, it means you add x.
Then a half as many he has, which means you add 1/2x.
Then a quarter as many he has, you add 1/4x.
Finally, if the boy also joins the school, it means you add 1 and obtain a total of 100 students.
Write this in an equation and solve for x:

The teacher has 36 students.