As you noticed, Destruction is pretty straightforward. Each cast and while continued casting, you will increase skill, but this has a caveat: only if you are attacking someone. If you're fizzling your spell in the middle of town, it's not going to increase it.
Alchemy is also straightforward: each potion created will increase your skill. As dpatchery notes, eating ingredients for effect determination will also increase your skill by a small amount.
With Enchanting, each enchant (or disenchant, as LessPop_MoreFizz notes) as will increase skill. dpatchery also notes that you can increase your skill by recharging items with soul gems.
Restoration, namely the healing spells, only increase skill if your health is below its maximum. The others only work if you're in combat or near hostile enemies: this includes spells like Steadfast Ward.
For summoning spells (essentially most things in the Conjuration school), you won't get a skill up for the summon until you enter the range of hostile mobs and, in the case of summoned creatures (like the Atronach and the Familiar), they do some damage. With bound weapons, entering range of hostile mobs is all that's necessary to gain Conjuration skill; however, using them in combat will increase their appropriate weapon skill, not Conjuration.
Alteration and Illusion spells that are targeted towards messing with hostile mobs need hostile mobs within range to skill up. Buffs, like Oakflesh, don't increase skill without hostility as well. Others, like Candlelight and Muffle, are recastable without hostility but only increase skill by a small amount.
Best Answer
You would need 100 in all skills and 89 in one, so 1484 skill levels (
[100 - 25] + 5 * [100 - 20] + 12 * [100 - 15] - [100 - 89]
).What's that, you want an explanation?
The formula to level up is:
if you remember your math classes and that whole
sum(1..n) = (n + 1) * n / 2
business, that means we needskill ranks to level 80 times (from 1 to 81). We start off with one skill at 25, five at 20 and the rest at 15. Leveling each skill to 100 gives us
points, which brings us to 4725, 4840 and 4930 skill levels, respectively. If you level up all skills except one fully, we're at 83155 (
4930 * 11 + 4840 * 5 + 4725
), so still 3845 skill levels short. So with that last skill, we can solve for the the final level required:Note:
Using a major or minor skill here wouldn't affect the calculation:
Also, you can test it out with an online character creator, like this half-baked one I threw together.
Edit:
Semi-interestingly, if you want to know the maximum number of levels required, it's only one more than the minimum. It would involve 8 skills at 100 and 10 skills at 99, so 10 less than the maximum number of skill increases available versus 11.
Edit 2:
In case anyone's confused about why I'm ignoring major/minor skills or anything else that might have affected a character in Morrowind/Oblivion, there's a great answer on this topic. All skills are born equal. Leveling a major skill Sneak from 30 to 31 is the same as leveling a standard skill Sneak from 30 to 31, which is the same as leveling a minor skill Archery from 30 to 31. All of them give you 31 points of "skill level XP" or whatever we want to call it. The bottom line is that these mechanics are understood and the above answer should be true for all races and all skills.