All other wall spells are conjuration - the spell description even starts with the word "summons". Is it just because there are no high tier illusion spells for single class illusionists to take??? Will this be fixed in DLC???

I'm pretty sure it's meant to be illusion - while other walls like fire and force have a physical presence, spells that manipulate light and color are typically under the illusion school (e.g. Arkemyr's dazzling lights). That said, the fact that the wall applies elemental effects gives it a physical component and complicates things - for an illusion spell, it should probably generate random illusion effects like charm or confusion rather than (or in addition to) physical damage. 

The whole specialist mage thing is pretty badly implemented, imo - too much cost for too little gain. I would be surprised if there aren't other questionable/debatable keywords on the spell list.