I'd like to suggest that instead of an increase to the duration of a summoned creature, that wizards who take the school of Conjuration at character creation instead get to cast summoning spells at +1 circle. (Casting "summon creature I" would instead be "summon creature II", and so forth)
Additionally, for summon swarm, wizards of the conjuration school could perhaps summon (2-3) + 1 creatures at the first circle.
As someone who has played a conjuror before, i;'m sorry to say no. Absolutely not, no.
+1 Spell circle is /insanely/ powerful in some cases and wizards already get conjuration 5 at level 9 with no restrictions. Otherwise to get it you have the problems of people hunting you of being a druid, the need to get a level 5 relic for a cleric, or needing to hit level 10 for being a sorceror.
Wizards are already one of, if not the best conjurors in the game and this would brokenly unbalance to an already very strong archetype.
It would be cool, but I'd probably have to agree with Dredi here. There are already ways to boost your summoning up by a spell circle that do not require DM loot, and I would fear that this would either result in Conjuration wizards becoming disproportionately strong, or nerfing summons to counter it. Honestly, I'm pretty sure our current standard of having DM's gate specific themes and consistent +1 stuff is better for balance.
As a frequent summoner player, I'll also chime in to say this would be one of the most overpowered ideas that has yet been suggested here. As has been noted, there are already ways to boost the power of a summon by 1 circle, and having these boosts stack would make summoners just game-breakingly powerful. Even if they didn't stack, this would still easily make spell focus conjuration the strongest feat in the game, with how strong some of the custom summoning themes are.
A better idea in this direction would be to have a 1/reset (2/reset for GSF) consumable for the feat that could be used to boost your next summon (or few maybe) by 1 level. That's still much stronger than the current feat though, which is already a good one that is frequently picked.
What about, instead of upping the summoning spell, taking the spell school: Conjuration would give summons increased hp or temp hp that scales as: level * (circle from which the summoning spell originated )/2 . Example: Casting summon creature 5 and being a level 10 would give the summon 25 temp hp.
Summoners are pretty powerfull as they are. They dont really need a buff.
I would have to agree unfortunately.
If you feel wizard doesn't get enough for the loss of its transmutation spells for selecting conjuration i'd suggest simply switching to another wizard school and taking conjuration as your spell focuses instead. Conjuror is really quite strong though it can fall off a bit at the highest levels of play/content, its still extremely powerful.