When Russel ditched Annie in the hallway, he did not run to the nurse's office. He ran to the boy's bathroom.

It was safe there, you see. And he could clean himself up and hide the evidence before anybody else saw.

The bleeding had stopped by the time he got to a sink, but there was still enough blood on his hands and face to make it pretty obvious he'd just suffered a severe nosebleed. That, or to make someone suspect he'd just murdered a person.