Line two sheet pans with parchment. Spoon yogurt into 24 small dollops and space banana slices beside them; freeze for 3 to 4 hours until solid.
Divide frozen yogurt, banana and berries among six freezer bags, using four yogurt dollops and 1/2 cup berries in each. Flatten and seal, label with the date and freeze for up to 2 months for best quality.
To make one serving, put 3/4 cup refrigerated milk and 1 tablespoon peanut butter in a blender. Add the entire contents of one frozen pack.
Blend until completely smooth, stopping to scrape down as needed; let the ingredients stand in the blender for 3 to 5 minutes if the motor struggles, then blend again rather than forcing it.
Pour into one large glass and drink promptly, or refrigerate at 40°F or below and use within 24 hours. Keep the milk and peanut butter separate from the freezer bags until blending.