jag said:March into your deans office- this was an innocent mistake and you should not be penalized by having something like this on your record. Offer to pay restitution to the school in exchange that no charges, informal or otherwise, be brought against you. Don't be snide, just explain that this was a stupid mistake and you had no intention of stealing anything, you simply thought this was trash- it is afterall an empty cardboard box. Explain that you aspire to go to graduate school (even if you don't) and something like this could ruin your academic career (even if it won't). You would be happy to apologize to the store manager or whomever (with your best fake smile) to make ammence.
Good luck~ I am sure it will all work out!!! We are all here for you!!!
Thanks, Jag!