It's not a proper sentence first of all it should be why did the elephant go in the boys bathroom, but needless to say he probably went in to take a elephant size dump.
Yes, the bathroom is for all boys, so the noun is the plural form. The bathroom for the boys is the boys' bathroom.
You use the boys bathroom.
One example: That they all went to the bathroom in one designated area.
Boys go to the bathroom by standing up and holding their penis.
Because he wanted some nuts
He went to the bathroom.
they went to the bathroom and then he went to the bathroom.
Because he had to go pee ;) ~kayla :)
I heard that you went to the beach on an elephant!
they went to the bathroom in a stream that they did not use for drinking water.
Not sure
Because it's cool to watch us boys