To go along the line I mentioned in the last comment, here are the ways I KNOW we can emulate some of the special abilities from a school:
Increases to a non-combat skill: Simply buy a stunt relating to it (i.e. Trick Riding: Gain a +2 on rolls regarding Perform while riding a horse).
The ability to do X: Buy a stunt that allows this to happen. Shield use, mounted combat
Negate the penalty of. . .: Some of this is narration (two weapons, for example), some is mechanical (the aforementioned shield use). We should be good there on a case-by-case basis.
Two Attacks: The Two-Weapon Fighting Stunt takes care of this problem.
Attack Multiple Targets: The Wall of Death stunt for a spray attack can cover this without an issue.
If X, Then Y: The biggest examples of "If your opponent misses, make a free disarm attempt", "If you bind your opponent's weapon, you may attempt to disarm them immediately", or "If you bind, gain a bonus to disarm." Each of these can utilize a current mechanic or be a stunt, as they are all under a VERY narrow focus; the miss can use the new boost mechanic from Core (stunt to reduce it), same thing with the Bind, and the disarm bonus can simply be a stunt.
Gain a Fear Rating: Two approaches to this: buy a stunt to gain a bonus for Intimidation, or use Weapons as an Intimidation attack/maneuver. Could even buy both and it should work.
Free Raises: Simply compels or buy a stunt that gives a bonus of +1 for using the weapon in that appointed matter.
New Defense: Anything that allows you to roll to set a new TN for your defense should simply be read as a Block using the Weapons (or related) skill.
Combat Bonuses: This is aimed at things like grapple, off-hand parry, horseback archery, etc. I'd be okay with buying a stunt to grant the +1 bonus aimed at this.
Interrupt Attacks: See the notes on Stop-Thrust in the previous post.
Left Handed: This tends to be a big thing in the fencing world, and a few schools give this bonus. Honestly, I'm game for invokes here, as a stunt would be too much potential for stacking.
Drunken Boxing: I think a stunt is perfectly acceptable for this, as it is rather limited with how you can acquire that bonus.
Improvised Weapons/Shields: I think the declaration, assessment, maneuver, and even just the Weapon/Fists skill covers this rather well, and will leave it like that.
Gelingen Monster Hunting: Use the Occultist stunt to help with Declarations, Assessments and Compels. This may be an alternative to general Exploit Weakness.
Now for the stuff I can't decide:
Attack and Defense bonuses: Most of this can simply be invoking of the aspect with the school, but since the source material has it steadily increase based on rank, I have a hard time accepting this.
The closest thing I can think of is to provide a stunt that grants a +1 to attack or defense within certain circumstances (while using that weapon, only in melee), and allow it to be purchased up to three times.
Damage Bonuses: There are a few ways around this. The first is the rip off the effect of Inhuman Strength when it comes to damage, and only with the weapon in question. Another is to use a stunt, like a weakened (non-FP powered) Killer Blows to increase the Weapon Value of the preferred weapon by 1.
Alternatively, there was a stunt in Fate Core called Killing Stroke: if your opponent is about to take a consequence, spend an FP and force them to take a consequence larger than planned (i.e. if they took 6 stress and wanted to fill their -4, you could spend a FP and make them fill their -6 instead).
Finally, basic compel/invoke mechanics could, in theory, work.
Weapon Breaking: There are multiple thoughts here. The first is to make the break like the grapple rules or a disarm; set up your opponent by creating an aspect to invoke, then invoke it for the purpose of your task. Basically it removes the weapon, only more permanently.
Alternatively, you could give the weapon a number of stress boxes/durability and have it absorb the abuse that way.
"Focus" Dice and similar effects: Some schools grant a bigger dice pool for set tasks (like Initiative), or even ANY task involving combat (like the bonus from Aldana). I'm going to be honest: I don't know what would be best emulate this. Modular Abilities? The ability to purchase powers (which gives them an INSANE edge)? New stunts?
If your opponent misses, their defense drops: The Riposte stunt as written is the closest thing I've seen to this. Any better ideas?
Extra Actions: I really don't think DFRPG/Fate is up for multiple actions per turn without taking penalties, especially considering some of these are multiple attacks. . .
Multiple Attacks: Dealing with a two weapon school with two attacks is easy to deal with. Dealing with a two weapon school that allows MORE than two attacks is the challenge. Perhaps further stunts to increase the damage from Two-Weapon fighting?
Lower Opponent's Defense. Period.: I'm looking at maneuvers again, because making an attack that is harder to parry seems like a lot of tags, invokes, and compels. Having a stunt that always causes that drop seems a bit to OP, and it wouldn't be anything more than a +1 attack with the way stunts are set up. Opinions?
Bonus Drama Dice if X condition is met: This would be the equivalent of giving out free Fate Points for the scene. I don't think this can be converted.
Skill X is now a bonus for Y: I can see this being an invocation for a bonus, but I can see that getting expensive. Perhaps a stunt that allows a secondary skill (like Perform) be able to modify/supplement the primary skill (like Weapons)?
Bonus Against Wounds: The closest thought here is getting a stunt that grants an extra consequence, an Armor rating or something similar. Basically, a weakened version of Inhuman Toughness. Otherwise, a stunt that allows Endurance to be used as a defense/block. Not certain what else to offer up.
Offensive Blocks: I know this tends to be bad juju in this community, but some schools (like Donovan) have the ability to deal a minor bit of damage after every successful parry. I can't think of a good way to do this, and would scrap it, but I'm looking for opinions.
Stances: Not many schools have them, and I'd simply just use it as navel gazing maneuvers or as a special variant of Modular Abilities, letting you purchase stunts (and ONLY stunts) based on the "stance" involved (one gives Initiative bonuses, one gives damage bonuses, etc). If this route is taken, I'd think reducing the cost of modular abilities from -2 to -1 may be in order.
Immunity to an attack: This is from one of the schools from Bryn Bresail, and I'm tempted to toss it. It's a contested roll, and if you succeed, you are immune to that attack for the rest of the scene. I think it's too close to the Immunity power.
Protecting Others: Would this constitute simply as a block, or something more?
Syrneth Tinkering: I know this isn't a "school" per se, but it technically is. The ability to tinker with ancient artifacts and make working devices almost falls into the purview of Crafting. . .
Raising the Stakes: With Valroux, if they call for a Raise (increased difficulty) and succeed, the opponent must match that roll at LEAST. Basically, it's a Block that uses the connecting attack roll for it's base.
A bit too OP?
Lower Defense For Stronger Attack: Some schools drop their guard to catch an opponent unaware and deal some major damage, especially with the Stop-Thrust. I don't think it translates well, thought.
