Alteration
Main purpose is protective, an alternative to armor for mages. It's not terrible at this, but it's also beaten pretty clearly by it's competitions as all significant penalties for armor are negated later on in the game while alteration spells will still cost a chunk of magicka and won't be on all the time like armor. Through perks, it also offers magical protection - 30% absorption and 30% resist magic is pretty nice. However, there's a large perk investment to reach those, and enchanting and other means of acquiring such seem like superior options. Paralyze is strong, once you get it, but Illusion offers superior crowd control options. Overall a pretty bland school and one I'd stay away from personally since pretty much everything it does can be done better. It has one gem of a spell however, an obscure one that luckily doesn't require any investment into the tree. Equilibrium converts health into magicka at a decent rate. This is great for training restoration for obvious reasons, but also simply a nice tool to have as there are plenty of occasions when that magicka is more important than the health it costs.

Strength/Usefulness 5/10
Design/Fun Factor 3/10

Conjuration
Conjuration is essentially a magical alternative or supplement to companions. It serves this purpose very well, with powerful summons and thralls to take the heat of you as well as dish out damage. A certain summon is probably overpowered even, I think any conjurers will know which I'm thinking of. You can go heavily into the tree for improved atronachs or undead and eventually two of them, but even without heavy investment you get some solid options with just the expert level spells. It can also bring you temporary magical weapons which don't really compete with the best smith improved weapons, but still hold their own pretty well and are especially great early game with the perk to improve them. Unfortunately, the bound weapons are limited compared to previous games. Still, easily my favorite school in Skyrim.

Strength/Usefulness 10/10
Design/Fun Factor 9/10

Destruction
It's name makes it's purpose fairly obvious, but unfortunately it fails to deliver for many players. It's damage doesn't scale and it's high level spells cost too much magicka. Some players report a positive experience or at least have been able to make it work for them using various tricks and techniques or other skills to support it, but I'd recommend passing on it unless you're adamant about shocking, burning, and/or freezing stuff which is entirely understandable. Depressing, considering they've done so much with the actual spell mechanics in destruction - placing walls and rune traps in particular I was hopeful about. It does have a secondary purpose however, perhaps not intended to be as powerful as it is, the impact perk is one of the best tools in the game for keeping enemies off the player.

Strength/Usefulness 3/10
Design/Fun Factor 8/10

Illusion
Illusion is mainly about control, but also a stealth supplement quieting your spells, your footsteps, and even making yourself invisible. The control aspect of the tree requires some pretty heavy perk investments, but will end up being very powerful for escape or avoidance, or for making enemies do your work for you by inciting them into violence against each other - which is amusing if possibly overpowered. A good school but demands a lot of perks just to make it's spells reliably function at higher levels, and probably too powerful once you have those perks. Kind of an "all or nothing" school outside of invisibility.

Strength/Usefulness 9/10
Design/Fun Factor 7/10

Restoration
Maybe not the most exciting, but certainly one of the most universally useful. Most if it's best perks, respite in particular is amazing, are very accessible in the lower part of it's tree/sign, and to do it's main job you don't need a heavy investment into the tree. It's non-healing spells however are pretty lackluster. Wards are a waste of mana, and the undead specific spells are, well, too specific to be worth spending perks to improve. Wards had potential to be an interesting mechanic to use in combat at least, but still a very lacking school. It's a one trick pony currently, though trick just happens to be one of the best. Unfortunately though, it's one trick is undeniably done better via potions, but I'm not sure I can dock restoration any points over this, since alchemy is obviously broken.

Strength/Usefulness 6/10
Design/Fun Factor 4/10