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Disclose. Just mention something like how you were immature and made a mistake; and that you’ve since learned from it, or something to that nature.
No, don’t give stupid advice.
OP, ask your dean’s office whether the write-up counts as official discipline for the purposes of law school. They’ll know what’s up. If they say yes, THEN and ONLY THEN disclose. If they say no, do NOT disclose.