Scouts is slightly different as they have their own insurance, they do allow the use of other stoves but often still move towards trangias, mainly because they're fairly complete and easy to set up.
If you're looking at the bronze award, in most cases the aim is to get the kids actually out and about, many of the students that I take out little experience outside of a home or school so its daunting enough to be camping and walking solo (ok supervision isn't far away.) Trangias are simple enough that they can get to grips with fairly quickly which is quite a confidence boost, even with this I've still seen some numptys manage to burn themselves.
I think when you get to silver and gold (gold especially) they already have some experience so the rules are relaxed some what, if the students can show profiency then I've no problem as a leader with them carrying and using them. Gold especially is another ball game, my other half runs hampshire stores as a part time job, renting out equipment to schools and groups, they're actually looking at buying some terra novas for gold groups ot use, so the rules here are quite different - normally for bronze they rent out space packers or vaudes.
Unfortuanately in this day and age you've got to cover yourself and a bronze group with no experience is much safer with a trangia and gas converter than with meths, green heat or some other stoves.
Red, if you get stuck or want any advice, drop me a message, I was out for 18 weekends last year with DoE groups
Matt