airwalkrr
Adventurer
This is such a simple idea that I imagine it has been thought of before. They might be a touch overpowered, but I will let you, EN World, decide. 
If they are too powerful, I believe a simple fix would be making prerequisites of caster level 12th and caster level 6th respectively. Bonuses to caster level are quite powerful at low levels but they diminish in importance at higher levels (with the "brilliant" exception of the holy word class of spells).
GREATER SPELL ENHANCEMENT [GENERAL]
Choose a school of magic to which you already have applied the Spell Enhancement feat.
Benefit: Add +1 to the caster level for all spells from the school of magic you select. This bonus stacks with the bonus from Spell Enhancement.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic to which you already have applied the Spell Enhancement feat.
SPELL ENHANCEMENT [GENERAL]
Choose a school of magic.
Benefit: Add +1 to the caster level for all spells from the school of magic you select.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic.

If they are too powerful, I believe a simple fix would be making prerequisites of caster level 12th and caster level 6th respectively. Bonuses to caster level are quite powerful at low levels but they diminish in importance at higher levels (with the "brilliant" exception of the holy word class of spells).
GREATER SPELL ENHANCEMENT [GENERAL]
Choose a school of magic to which you already have applied the Spell Enhancement feat.
Benefit: Add +1 to the caster level for all spells from the school of magic you select. This bonus stacks with the bonus from Spell Enhancement.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic to which you already have applied the Spell Enhancement feat.
SPELL ENHANCEMENT [GENERAL]
Choose a school of magic.
Benefit: Add +1 to the caster level for all spells from the school of magic you select.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic.