Possession of any graffiti implement by any person under the age of eighteen (18) years with the intent to deface property while upon public or private property shall be unlawful unless that minor has received the consent of the owner of such private property whose consent to such possession and presence is given in writing in advance or unless that minor is accompanied by and under the supervision of a parent or legal guardian. This provision shall not apply to the possession of felt tip markers by minors attending or traveling to or from a school at which the minor is enrolled, if the minor is participating in a class at said school which formally requires possession of such felt tip markers. The burden of proof in any prosecution for violation of this Section shall be upon the minor student to establish the need to possess a felt tip marker.