There are several information's already given in the question. Let us focus on them to get to the answer.
For every 70 children, the number of teachers required = 3
Then
For every 14700 children, the
number of teachers required = (3 * 14700)/70
= 630
The number of teachers in school now = 612
Number of more teachers required = 618 - 612
= 6
So 6 more teachers are required in the school.