Combine zucchini, sugar, pineapple, and lemon juice in a large pot over medium heat; bring to a boil and boil until zucchini is clear. Off heat, stir in gelatin mix until completely dissolved.

Meanwhile, inspect twelve 16-ounce jars for cracks and rings for rust, discarding any defective ones. Immerse in simmering water until jelly is ready. Wash new, unused lids and rings in warm soapy water.

Pack jelly into hot, sterilized jars to within about 1/4 inch of the top. Run a clean knife or thin spatula around insides of jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any residue. Top with lids and screw rings on tightly.

Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 5 minutes.

Remove jars from stockpot; rest, several inches apart, for 12 to 24 hours. Press centers of each lid with a finger to ensure lids do not move up or down. Remove rings for storage; store in a cool, dark area.