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?
First, we need to find the total number of students in the school. We know that there are w boys and x girls, so the total number of students must be w + x. 

Now, we need to find out how many more girls there are than boys. To do this, we need to subtract the number of boys from the number of girls. We know that there are w boys and x girls, so the number of girls must be x - w. 

This means that there are (x - w) more girls than boys in the school.

Therefore, the answer (symbolic expression) is: 

x - w