Depends. For "Generic Lightening Enchantment," Air is probably the way to go. For "Manipulate Material To Do What You Want Enchantment," I'd say Earth for metals, and maybe Water or even Necromancy for wood, leather, or bone.
NINJA'D:
I'm currently debating if I should give you the ability to make things lighter.
I mean, if I give you guys that ability, then managing weight becomes un-important as all you'll have to do is enchant things and there you go you can wear that Enchanted Tungsten Plate Mail for a lot less encumbrance points than you'd expect.
Also, I don't think air magic would affect the weight of an object. Heck, I don't think any school would affect the weight.
In terms of balance, I would tend to think the lack of ability (or difficulty) in enchanting it with anything else would help, since you can either wear tungsten armor for less than you should, or tungsten armor plus a shield enchant or what have you for full weight. That's not exactly apples to apples though, so that'd have to be your call.
In terms of feasibility, I would tend to think Earth magic could harden metals and such, and other schools might be able to do the same with other materials. Obviously nobody'd be wearing liquid water armor to begin with, but freezing it solid is still kind of a valid example.
Air could be argued to affect weight by affecting its buoyancy within air, in the same way that Water magic could potentially make something float in water. That assumes Water magic can actually do that, of course, which I'm not sure of.
EDIT: With talk of hardening steel and freezing water being relevant in terms of showing that the school can affect a material's properties, of course. The lack of Water magic's ability to melt ice weakens this, but still.