Half-and-half jokes

Two artists had an art contest. It ended in a draw.

A teacher asked his students a math question.

"You have one dollar. Your parents give you five dollars. How much money do you have?"

After some thinking, about half the class raised their hands. The teacher called on a little girl in the front.

"One dollar!" she said.

I saw identical twins. I threw a paper plane at them.

A friend of mine just got divorced. He and his ex-wife split the house. He got the outside.

Why is it that when I'm in school doing PE, it's fine for someone to say "boys against girls", but the moment I say "blacks against whites" I'm the bad guy?