There is none.
Technically speaking, you are not so much casting an evocation spell, but rather, an illusion spell who effects just happen to mimic that of an existing evocation spell. In theory, each school offers a unique feature. But in practice, you find that some schools readily step on the toes of other schools because they contain spells which can replicate effects once thought unique to other schools (for example, conjurers make decent blasters because of the orb spells).
Though things get more ridiculous when you realize that the shadow evocation/conjuration line of spells have advantages over and above that offered by evocation/conjuration spells.
For instance, the former have a casting time of just 1 standard action, regardless of the actual casting time of the original spell. Thus, phantom steed requires 10 minutes to cast, but just a standard action if replicated by shadow conjuration.
Next, they ignore any material component/focus. Yep, forcecage without the need for an expensive diamond. Contingency without requiring a statue as a focus. If you play a shadowcraft mage, you can replicate apocalypse from the sky without having to first procure an artifact. Oh, and the casting time is 1 standard action, instead of 1 whole day...
Then, this is where you retort "Ah hah! They have only a limited effect if the foe makes the will save to disbelieve. So a shadow fireball can never replace an actual fireball". Then I point you to the shadowcraft mage build, which cranks the %effect of the shadow list of spells to past 100%. So what really happens is that the spell has a 100% effect if foes fail their will save, and a 120% effect if they succeed on their save! Yes - you actually want foes to be making their saves...
I wouldn't call it a loophole. More of a design oversight. No one doubts that the shadow line of spells work the way they do. The bigger question is whether they should (eg: should illusionists be able to spontaneously convert silent image to replicate any conjuration: creation/summoning and evocation in the game?).