My experience is that most kids are fine being tested without warning. Some are actually better that way, and a small minority need forewarning. I've had parents tell me not to warn their child, because they would be up all night worrying about it if I did.
And first thing in the morning is much better than in the afternoon, for the majority of kids. It also has the advantage that you don't have to be ostentatiously pulled from your classroom in front of everyone.
The reaction of the child partly depends on the school culture; if kids are often being pulled out for this and that, it doesn't feel as strange.
Now if -you- didn't know that an IQ test was going to be administered at all, then that would be inappropriate, as parental consent is required.
