According to the conditions in the question, the freshmen can be divided into two parts, one is as many as the sixth grade, and the other is 20% more than the original number of the whole school.

So, 350 freshmen are equivalent to the original students of the whole school:

(20%+ 1/6) There are more than 20 people.

So all the students in the school have:

(350-20)/(20%+ 1/6)=900 people.

Method 2: Equation.

Suppose there are x students in the whole school,

Then the number of students graduating from grade six is: 1/6X+20.

The new first-year students not only occupy the vacant positions of sixth-year students, but also increase by 20% compared with the original school.

So the new students are:1/6x+20+20% X.

So we can list the equation: 1/6X+20+20%X=350.

1 1/30X+20=350

1 1/30X=330

X=900