Have a basket or cubby or even just a plastic dishpan for each boy. Choose different colors for each boy, and label them well in permanent marker. When each boy comes home from school, every lunch box, water bottle, hat, coat, a paper that needs to be signed, etc. goes in the proper basket. You will know exactly who brought what home. And who didn't. And in the evenings, pack the basket with each boy's supplies for the following day: coats, a filled water bottle with ice in it, the permission slips, project or notebook, backpack, etc. (everything but the lunch box if the food needs to stay cold). Teach the boys the system: prepare the basket in the evening, take everything out in the morning on the way to the bus, and return everything immediately after school. Review the plan and the consequences and the reasons (expenses, being responsible and trusted, etc) for the new plan. Some kids are very visual, and you can post the rules and the system on a whiteboard or posterboard above the baskets. Get the boys involved. Have them print out the rules and decide the consequences for failing to adhere to the system.
Make consequences. Say "Johnny, I notice your basket is missing _______ (bottle, backpack, hat, whatever)" or "you didn't return your things to your basket. Instead you dropped them on the floor or scattered your things everywhere. You will have this punishment as a consequence for not taking care of your responsibilities:__________ " (and then make the punishment fit the crime. Make him clean the bathroom or pick up all the Legos or wash the dishes or clean the oven -- some chore that reflects caring for our belongings. That's a first offense. Second or third offenses can mean loss of tv or video games, or something that means a lot to him.
And don't be afraid to tell them the cost of a new coat or replacement backpack. Maybe give them each $25 dollars or so and have them shop for their own supplies from their school lists. They might be horrified at how much stuff costs!
I made my son sign a contract when I purchased something expensive for school. I was ok with a particular backpack, but he wanted a "cooler" one that of course was more expensive. But he had lost one not long before. So we wrote out a personal contract. We noted the price of the backpack of his choice, and stapled the receipt to our contract. The deal was, and we put this in clear language on the contract, if he kept it for the school year and didn't lose it or destroy it (other than normal wear and tear and scuffs, of course), I would pay for it. If he lost it or ripped it due to carelessness, he would have to fork over the entire price of the backpack. I did that for an expensive calculator too. And I laminated the contract that we both signed, and posted it. He didn't lose his backpack or calculator! I duplicated the contract and kept one by his desk, one in his notebook and one in the kitchen near the calendar.