How can I best explain to my sons teachers without getting into trouble?

No I'm not trolling I promise. My 9 year old is a massive Star Wars fan and I buy him Bladebuilders lightsabers which are toys that let you build your own lightsaber. Well he loves having lightsaber fights against me and we have them a lot. Well yesterday it went wrong and I accidentally bashed his face. He... show more No I'm not trolling I promise. My 9 year old is a massive Star Wars fan and I buy him Bladebuilders lightsabers which are toys that let you build your own lightsaber. Well he loves having lightsaber fights against me and we have them a lot. Well yesterday it went wrong and I accidentally bashed his face. He wasn't crying but there is a red mark. I would NEVER hurt him on purpose and I was horrified. I'm scared I'll get in trouble if I say why he has that mark on his face to his teachers. In my defence though everyone knows he loves Star Wars lol. I feel horrible. And I know I should be more careful. 

Btw please no saying I shouldn't have lightsaber fights with him. All kids deserve some fun. However I'll be extra careful next time as I feel terrible. Anything I can do to get the bruise out of his forehead by tomorrow?
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