I'm applying for college with an open display of alcohol charge

Over the summer, my friends and I were at the beach in New Jersey. We were sitting in a congested area with many people around us, however, some people left their cans of alcohol near us and the police wrote us tickets, thinking it was ours, but it wasn't. They wrote us two tickets, one for underage drinking/possession, another for open display of alcohol. We went to court and explained the situation to the prosecutor, who dropped the underage drinking charge, but said we had to plead guilty and pay the fine for open display of alcohol in order to get the underage charge dropped. We agreed because it was the only viable option we seemed to have and paid the fine, but I'm concerned. The school I am applying to is asking if I've been convicted of a crime, and if I entered a plea of guilty under a first offender act. I'm not sure of these terms and whether or not open display of alcohol is a felony or conviction, and I don't want to falsely report anything to the school.