It can't be conjuration because: "Conjurations bring manifestations of objects, creatures, or some form of energy to you ...or create objects or effects on the spot... " Neither holds for conjuration. Evocation, however, "create something out of nothing."
I don't think Transmutation fits because you're not modifying the properties of the target creature. You may be modifying your 'thoughts', but the target is what's important. I suppose you could say that you're modifying the target into being able to receive the sending, but that's shaky at best. You'd also need to create the sending and modify yourself to send it. Consider message on the other hand. You whisper a message normally, but you are modifying the target recipients to be able to hear it. The spell doesn't actually create the message.
As for universal, that's actually no school. I personally like the idea of a universal school, but it was not implemented that way and there's no (good) definition for it. It's basically a list of spells mad available to all casters, regardless of specialty.