You found a supply of plutonium the museum was using to make glow in the dark paints! That doesn't sound safe, but hey, it's the 80's.
Now all your team has to do is make it back to the Delorean.
To make it back to the car, solve the last puzzle found in Saturday Detention. Check the library for more resources to help you out (it's also a great place to start a sharing circle).
Once you have followed the instructions from the last puzzle, check the Home Screen to reveal the final step.