Spread the peanut butter on half of the bread slices, keeping most of it in the center, especially the jelly. Make as many as needed.
Seal & remove the crusts:
Push down from the sides, like using a cookie cutter, and once you've cut through the bread, press the middle sealer down and hold it in place for 15 to 30 seconds to seal the edges. Cutting it first, then sealing, ensures a tight crimp and helps prevent the filling from leaking. Holding it down will seal it tightly and prevent the filling from leaking. After that, pull the crusts away from the sealer.
Freeze for later:
If making a lot, place them on a parchment-lined baking sheet and freeze. Once frozen, transfer them into a large zip bag and keep them frozen. Or, place each uncrustable inside a sandwich size zip bag and freeze individually.
Pack for lunch:
Pack the frozen uncrustable inside the main compartment of a lunch box. Add any sides, like fruit and veggies, in separate compartments or containers. If the uncrustable is inside a zip bag, just add it directly inside the lunch bag.