A spellbreaker inquisitor that picks for the extraordinary ability impervious the school conjuration is "immune to the effects of" spells of the school conjuration, and, "[i]f a spell of that school is an area of effect spell, the spell goes off as normal, but the Spellbreaker [sic] is untouched by its effects."
There no way to know what this really means short of asking the author, the archetype spellbreaker even going unmentioned by the Ultimate Combat FAQ. The GM must make the call.
That is, many spells—including, for example, the summon monster spells—have Effect entries (for details see Aiming a Spell), but whether or not the level 20 spellbreaker inquisitor should actually be immune to all these proper Effects—whatever immune means here—is unknown. So I'm clear, besides its impact on summon spells, immunity to spells of the conjuration school raises other questions: Can the spellbreaker inquisitor enter the effect of a mage's magnificent mansion spell? Can she ride the effect of a phantom steed spell? If she throws an acorn grenade that's created by the spell fire seeds, does it explode? If a successful Use Magic Device skill check enables her to cast from a wand the spell fire shuriken, would the spell fail? Read even more broadly, does being "immune to the effects" of spells of the picked school include immunity to ancillary and collateral effects of such spells? Like, if a house is swept up by the effect of the spell tsunami, and the house hits the inquisitor, is the inquisitor untouched by the house? (Note that I don't need answers to these questions; instead, they are the kind of questions my inner editor would've raised were I to've homebrewed the special ability impervious. I'd've rewritten the special ability so that the answers are, respectively, clearly Yes, Yes, Yes, No, No, No.)
This reader suspects—yet can't prove—that the special ability impervious is supposed to be like the special ability magic immunity that's possessed by some golems—making the special ability impervious, essentially, unbeatable spell resistance that can't be lowered against, in this case, spells of one school of magic—, but, for instance, the special ability impervious grants immunity even to spells of the picked school that possess the entry Spell Resistance: No… which is utterly unlike the special ability spell resistance and the special ability magic immunity. This makes this reader hesitant to draw parallels, especially since the author of the archetype spellbreaker didn't see fit to either.
What this GM would do
First, I'd sit down with the spellbreaker inquisitor's player and have a very serious talk about what being immune to spells of the conjuration school means to the player and the campaign. A level 20 PC that can't be affected by spells of the teleportation subschool is likely to soon find himself a victim of the vagaries of mundane travel (yuck!) and miss out on adventures as the rest of the party bamfs from locale to locale. Similarly, considering the horrible things that routinely happen to high-level PCs, a level 20 PC that can't be affected by spells of the healing subschool—and that includes, like, raise dead, by the way—just isn't long for the campaign. (Headstone Idea: "She picked conjuration. Whoops.")
Then, if the player still refuses to reconsider, we'd negotiate a short list of how the special ability impervious works in the game. At the top of that list, for instance, I'd put that the special ability impervious when applied to spells of the conjuration school prevents attacks by summoned creatures in a way identical to the protection from alignment spells do. Getting a 1st-level spell effect from a level 20 capstone seems reasonable to me. Then the player might propose the whole house thing I mentioned above, and I'd nix that, but I'd rule that the ability didn't affect ongoing effects, so the PC could still toss acorn grenades, ride a phantom steed effect, and enter a mansion effect. That sort of thing.