ANSWER and EXPLANATION
We are given that all classes have a limit of 20 students.
If 35 students are going to be added, then, there must be at least 35 spaces in all the classes to accomodate them.
To find the amount of space left in the school, we have to first subtract the number of students in each class from the class limit and then add them up:
As we can see, there are only 32 spaces left in the school.
Therefore, there is not enough room left for 35 more students.