This is a stupid idea, but you can try drawing on your arm with marker and just saying, "I was bored," as a reason. But, I agree with NoOneLivesSuchAnEasyLife. Though, I would understand if you wouldn't want to do that. I'm sorry if I wasn't much help, but I wish you the best of luck!
You may gonna reveals your secret. Im sure cutting is your biggest secret from everybody. There is really nothing to help out. If you did a long sleeves. Teammate would think something different in you. Just show them your secrets. Some support. Some will leave. People will also treat you differently