Backpacking light with scouts is a challenge. For one thing, scouts are pretty hard on their gear. Scout age kids tend to be pretty neglectful of things like packs, tents, and clothing, so ultralight equipment and clothes don't last long when used by scouts. Anything durable enough to survive use by scouts is unlikely to be ultralight.
Also, I've found that most scouts' parents aren't all that excited about buying expensive lightweight sleeping bags, jackets, etc. for their boys.
After 2 decades of taking scouts on backpacking trips, I go for a "sort of light" approach. I weed out the criminally heavy clothing and equipment, and try to guide the boys towards lighter weight options.
Here is a copy of the gear list I give to parents ahead of our annual week-long backpacking trips in the summer.
It is not really an ultralight list, but it does help to give the parents an idea about what to bring.
Even with the list, the boys try to bring some truly useless stuff on these trips, so I have pretty rigorous pack checks ahead of time to weed out the excess gear.
As far as shelter goes, we just use regular 4 man tents (Kelty Gunnison). Split the weight 4 ways, and it's really not that bad. Colorado and Wyoming can be pretty mosquito infested, and it's good to be able to give the boys protection from bugs, even though tarps would be lighter.
I use a Coleman Peak 1 Xpedition two burner stove, and bring a couple of large pots and an aluminum griddle for cooking trout.
One thing that has really saved weight and also cut down on the task of pumping water is that we tend to use Katadyn water purification tablets for most of our water purification needs. I typically bring several big Platypus water bags, fill them in a stream and leave them to purify overnight with the tablets. Much lighter and less work than pumping water for a big group.
Here's the list I use for summer backpacking trips in the Rockies:
Necessary Equipment: Anything not on this list must be approved by me. This is a long and very strenuous trip, and only the essentials should be carried.
DO NOT BRING iPods, radios, gameboys, or other useless stuff. Also, because we will be out for so long, more than 10 miles from the trail head, all of this equipment is necessary. If there are some items you don't currently have, I can let you know where to obtain them at the lowest cost. I may also have items which may be borrowed.
Water bottles (2 quarts total) A “Camelback” or similar bladder is also ideal.
Flashlight A small flashlight such as a "mini mag light" or a small headlamp is best. Do not bring a big, heavy flashlight. If you conserve batteries, an extra set of batteries will not be necessary.
Chapstick and sunscreen
Insect Repellent. NOT a metal aerosol can. Small squeeze bottle or spray bottle. Best is Ultrathon brand.
Map (we will provide the boys with maps of the area)
Backpack Must have either an internal or external frame, and a padded hip belt which will support the weight on the scout's hips, NOT his shoulders.
Sleeping bag (NOT a big heavy bag. Should weigh no more than 3 pounds.)
Closed cell foam sleeping pad or ultralight 3/4 length self-inflating (i.e. Theramarest) (Do NOT bring a blow-up air mattress)
Plastic or aluminum bowl (Don’t bring a big army mess kit. All you need is a bowl.)
Small tube of toothpaste (get a trial size tube)
Hand Towel (A rayon towel, such as are sold for scuba divers and household cleaning is a good alternative to a regular cotton hand towel, as they absorb more water when wet, and dry quicker than cotton, and are lighter.)
Toilet paper 1/3 roll in a zip-lock bag (An even better option is a travel pack of diaper wipes.)
2 ounce bottle of liquid Soap (camp suds, or similar biodegradable soap)
Zip lock bag containing the following: 4 Johnson & Johnson Band-Aid Blister Relief pads; 12 aspirin tablets; 4 large band-aids, SMALL tube of Neosporin ointment.
Duct tape wrapped around a half a pencil.
Baseball Hat or other sun hat
Warm sweater or warm down jacket. Don’t need a huge ski coat here. NOT a cotton sweatshirt, jean jacket, etc. Wool or synthetic or down sweaters or jackets only.
Rain jacket with hood
Short-sleeved shirt(button up with collar is better than t-shirt)
Swimming suit (also to be used as shorts for hiking)
2 pair underwear
2 pair wool hiking socks NO COTTON SOCKS!
Hiking boots or running shoes.
Light-weight camp shoes (teva sandals, aqua-socks or extra running shoes) These shoes will also be used for stream crossings.
Synthetic or wool Long john top and bottoms (these will be worn for sleeping; evenings; and for cold days. NO COTTON.)
Hiking pants Nylon warm-up pants or similar are ideal, as they dry quicker than cotton. Pants should fit comfortably under pack's waist-band, so warm up pants are better than pants with belt loops. A light-weight alternative to pants is to bring a pair of shorts and wear your long underwear bottoms under them when leg protection is needed.) The very best option are pants with zip-off legs that double as shorts.
Snacks: Some between-meal snacks are a good idea. Bring something that won’t crush or melt or go rotten.
Fishing gear (GO LIGHT. No need to bring everything. ) 16 or older also needs license.
Lightweight liner gloves (if you have them, they can be nice on cold evenings)
2 pair synthetic liner socks (if your feet are prone to blisters, these can help)