What about one double hook for each cubby?

Or a vertical double instead of the horizontal could work well.

If you are doing the double hook I'd definitely just do one hook per "locker."

Oh yes, each of our lockers is about 24" so 15" would definitely be too narrow for 3 hooks. Even 2 hooks might look crowded with coats on each of them; you'd be surprised by how much space a jacket takes up. But if you're looking for functionality over looks, the more hooks the better :)

Could you put 2 hooks high (for coats) and 2 lower (for scarves, hats, etc.)? If you are concerned about the hooks looking crowded and messy you could paint the hooks the same color as the cubby walls. Two double hooks per side would give you 12 spots to hang things in each cubby - the odds that they hit the hooks are increasing!

Whatever the spacing I would add a trim piece of wood, 1 x 3 or so, on the wall to put the hooks on. The hooks won't pull out of the wall and you can change the hook spacing easier if needed.

I think two hooks would work, but I bet that once you get it installed, it will become more apparent what you should do. I think you can successfully get everyone to hang up their things. They just may need "incentives"... I wish we had a spot like this. I have installed hooks in the hallway (it is a square, not a long hallway) because there is nowhere else to put things. I keep my coats in the coat closet, which is too tiny to fit everything.

Oh, and I also agree with wacokid to make sure the hooks are put into wood for sturdiness. Backpacks can be heavy.