In a school there are w boys and x girls. y more girls joined the school. How many more girls than boys does the school have?
There are w boys and x girls in the school. y more girls joined the school, so the new total number of girls in the school is x + y. 

The new total number of boys in the school is still w. 

This means that the school now has (x + y) girls and w boys. 

To find out how many more girls than boys there are in the school, we need to subtract the number of boys from the number of girls. This gives us (x + y) - w. 

Therefore, the answer (symbolic expression) is: 

(x + y) - w