When you add up all the girls from all 4 classes, you had 12+15+9+10= <<12+15+9+10=46>>46 girls
When you add up all the boys from all 4 classes, you had 13+11+13+11 = <<13+11+13+11=48>>48 boys
There are 48 boys and 36 girls so 48-46 = 2 more boys
#### 2