Ask your manager, or just show up with a long sleeved T-shirt under your uniform. My husband is doing a (prayerfully) shiort stint as a fast food manager. (He’s got a Master’s of Divinity degree and is on leave from call as he/we discerned what to do.) At his business they are permitted to wear jackets of the same colorof the work shirt.
If need be, concoct a half truth, saying that you were outside playing over the weekend and you got some cuts you wanted to cover up for hygiene purposes. Also, put ointment on them so they heal quicker.
(I understand self-harm– physical, and emotional. I’ve lied about bruises and wounds. However, I don’t condone it.)