Flexibility is key. [Feat]

[Triaxx]

Flexible Mind
Prereq: Caster level 3rd, Spell Focus (Any)
Effects: Flexible Mind gives the wizard the ability to apply a +3 to concentration checks when casting spells of the focused school. Greater Spell Focus adds and additional +3, for a total of +6.

[Macrovore]

that's really weak. just take combat casting or skill(focus) concentration.

[Triaxx]

It is, isn't it? It's supposed to be weak, but it's also going to be part of a larger family of feats. There's four of them that go with it, but they need to be balanced against the two you mentioned.

[Miles Invictus]

Increase the bonus to +2 or +3, and you're good. It fills the specialist niche without overshadowing the more general concentration feats.

[Triaxx]

As a +2 from Focus and Greater Focus?

[Miles Invictus]

Yeah. So if you take both, your bonus is +4.

I really think you could get away with a +3 bonus per feat and have it balanced.

Consider: You have Spell Focus and Greater Spell Focus, and you're going to boost concentration. You can either take Flexible Mind, and get a +4 to Concentration checks related to one school, or you can take Skill Focus (Concentration), and get a +3 to all Concentration checks.

Unless you know you're only going to use one spell school your entire adventuring career, it's a no-brainer to take the Skill Focus feat over the Flexible Mind feat, because you can use it in more circumstances.

Now, if the feat gave +3 per Spell Focus feat...that's significant. +6 to Con checks with one school, versus +3 for all Con checks? It's a tough decision, influenced heavily by your spellcasting habits. Which means both feats fill a niche very nicely.

[Triaxx]

I envisioned this, for a friend who uses a druid with Spell Focus (Conjuration), and Augment summoning. Being able to take this, along with Skill Focus, gives him the ability to make certain even if surrounded he can have a chance to drop a last elemental, or other summon before falling. But you make a good point. I think +3 is about right.

Should this be taken once for each school? Or one time but applied across all schools for Spell Focus?

[Miles Invictus]

Concentration is not a particularly powerful skill, and feats are precious. You're not going to break anything if you only need to take it once.

[Triaxx]

Thanks, I think it'll work very well.