Most of the scouts in my area use externals, but its because they are half the price, very sturdy, and more adjustable - which is important for parents who want a pack that will fit a 12 year old, and still fit a 16 year old later after they grow.
The scouts who can afford to, switch to internal frame packs after they get close to an adult height (and their parents are pretty sure they won't have to buy another pack next year). Part of the motivation may be to look cooler, but they do remark on how much easier they are to hike with on our trails (which are rocky, sawtooth hills with overhanging tree branches everywhere. Ie: big green tunnel :-) ).
In general, I would buy externals for a youth group like a college because you can adjust the width between the shoulder straps, torso length, swap out straps or hip belts when they get worn out or to fit different sized people, and they are more durable in a rental environment.
However, internals took over the market for a reason. Even though the externals were half the price, and are much easier to pack / live out of due to the big wide opening. ”Coolness” wears off quickly when something else works better, so I don't think internals were just more fashionable.
I personally think externals are great for hunters or people taking supplies to a remote cabin. Ie: who have to carry lots of weight. But for regular backpacking, I like internals a lot better. If I lived where trails were gently sloped, and didn't have lots of trees, then I might prefer the ventilation of an external.
That said, I did a lot backpacking over steep rocky trails with lots of trees with an external. So, if cost is a issue, get an external, drop the pack bag to its lowest setting if there are lots of trees or steep rocky trails - and enjoy your hike ;-)