Make a lot of Jell-O. Depending on how many students you have, you'll need 200 boxes or more.
This is a great thing to ask for in donations. About a month ahead of time, announce that you're collecting boxes of Jell-O for this event and set-up a collection bin.
Also, depending on your church and the size of your group, you may want to consider holding this as a children and teen event, rather than just one or the other.
It also works well to have a picnic-type lunch or dinner before this event. We buy pizzas and sub platters and ask for donations to help cover the cost. Desert is Jell-O, of course.
So, back to making the Jell-O. You'll want to buy store-brand, of course, but then make it by the bucketload. What I do is measure how many cups my bucket can hold and then divide that in half for a regular-size box of Jell-O. Pour the Jell-O in, add half boiling water, stir, then add the rest cold water. (a 52 cup bucket takes 26 boxes of Jell-O, 26 cups boiling water, 26 cups cold water. Note that the water ratio can be estimated. You just need enough boiling water to let the powder dissolve into.) Let it sit for an hour or two to cool down, then stick it in the fridge. If you put the bucket in the fridge while it's still hot, it just takes the refrigerator longer to cool it down.
Once the Jell-O has...well, jelled, you're ready to prepare your games.
Bobbing for Candy - scoop some Jell-O out into pans and add some Gummi Worms. Stir it around and have students face-plant into the Jell-O to dig out the candy. No hands. Just make sure you make a fresh pan for each student.
Jell-O Jiggler Eating Contest - make some of the Jell-O into Jiggler squares and give students a time limit (I suggest 30 seconds) to see who can eat the most.
Jell-O Marble Pick-Up - pour out some of the Jell-O into pans and then mix marbles into it. Students sit in chairs and pick the marbles out of the Jell-O with their toes, dropping the marbles into an empty pan. The person to pick out the most marbles wins. Replace the marbles for the next group.
Jell-O Slip 'N' Slide - Place a pile of Jell-O at the end of the Slip 'N' Slide for students to land in.
Jell-O War Finale - Pour the bulk majority of your Jell-O into a kiddie pool or two and let students have at, throwing it at each other and swimming in it. If you have the funds, once the Jell-O is pretty much gone, bring out an aerosol can of whip cream for each student and let them spray each other to add the finishing touch.
Take your hose from the Slip 'N' Slide and spray those kids, as well as everything else down. Give kids trashbags to wear or tell them to bring towels for the ride home.
Wash all your pans and other game supplies and store them for next year.