Wear a bandage?
Wear lycra shorts underneath your PE shorts and say your shorts are chafing you?
Wear different shorts than your PE shorts and say that the PE shorts are lost/ in the wash/ torn etc?

Plus depending on where the place is, the cuts might not be so obvious. People don't go round staring at your legs in PE, do they? It depends on what you'd be doing. If you were running, then it might not be noticed, but if it were basketball then people have more time to stand around and stare at people.