Story Related. This will unlock after beating Leech in a guitar duel and getting busted for PDA.
After Biff has told you about all the demerits, you need to find a way to get the PDA infraction. Start by taking the electric car to your house. In the garage, interact with "Marty's Guitar". Tell your father that you need it to impress Jennifer. Now head back to town square, and over to the soup shop. Knock on the window and see Leech and Jennifer making out. Walk in to the back alley and interact with Jennifer. Select the following dialog path:
"You and the SoupMo guy?"
"Guitar? I play guitar!"
"I can outplay Leech any day!"
This will initiate a guitar battle between you and Leech. This scene can play out a variety of different ways. It sometimes gives you different options, and sometimes Marty and Leech move differently, meaning you must react differently. Your goal here is to get Leech to stand on top of the plank above the dumpster, then hop. Do whichever moves are needed to get Leech into position.
Once Leech has fallen in the dumpster, Jennifer will be all over you. Watch the scene play out and you will earn more demerits, and a trophy.
Refer to "Final Solution" for the trophy walkthrough.