The unsimplified ratio is "33 girls to 12 boys"

32 girls to 12 boys

The ratio is "32 to 12"

But that is not simplified, because

both 32 and 12 can be divided by 4

Dividing 32 by 4 give 8

Dividing 12 by 4 gives 3

So the simplified ratio is 

8 to 3

The word "to" is replaced by a colon :

8 : 3

Edwin