Wow! If you will be tinkering with the bikes for an entire troop, you need just about everything, including some sort of course on bike maintenance and/or a good manual.

On the bike you have - the first worthwhile accessory I would purchase would be a pair of toe clips for the pedals (after the helmet). They are rather old fashioned, but they will improve pedaling efficiency immensely. They can be used with almost any kind of shoe which is not the case with more modern pedals. Toe clips can go along to the next upgrade.

If/when you want to upgrade, a quick and relatively easy path is the purchase of lighter, stronger wheels with aluminum rims and quick release hubs. They will make an immense difference in the ride of this bike and they can also go along to your next bike, if you stick with this same tire size.

Just a bit more about helmets, a subject on which I am a bit fanatical, having been present at more fatality scenes than I care to recall where a proper lid would have made a big difference. You will find an enormous price range in helmets, many of which will cost more than your bike. Look for the certification level - that is critical. As I understand it, all helmets certified to a given level all provide the same degree of protection. Anything extra you pay is just so much bling.

For carrying gear, I would heartily second a rack. Backpacks ride high, and mess with your balance, while gear carried low on the bike will actually stabilize it, especially if a front rack is used. I also recommend a waist pack when riding. It sits low and contains valuable items that go with me, even when I hop off. It even has a mesh bag for another water bottle.

I'll bet that very shortly you will be able to school all of us in bike repairs - a whole scout troop (good grief!)