When it comes right down to it the only reason to be so against Obama telling school kids to stay in school and work hard there is because of prejudice! Think about it, plenty of other presidents have spoken to kids without such criticism, and no other president, none, has been asked for their speach in advance!
I'm not so sure. No other recent Democratic president has addressed school kids (though I suppose there has been only one other recent Democratic president).
While there may be some portion of the radical right that has a racist motive in opposing Obama, my recollection is that there was also severe Clinton opposition starting from Day 1 that had little to do with his policies.
If Obama were white, might there be somewhat fewer people ranting at him? Perhaps. But I think there would still be considerable numbers doing what they're doing now. There are groups on both sides of the political spectrum who are so ensconced in ideology, so sure they are right, that those who disagree with them cannot be "real" Americans. A corollary would be that if a majority disagree with their positions, they must have been misled or brainwashed, hence the fear of further "indoctrination" by the leader.
I'm willing to accept the prejudice explanation for some portion of the dissenters, but I'm not willing to speculate how widespread it may be absent evidence to do so. And given the vitriol launched at Clinton by some groups that for reasons that lacked any real connection to his policies, I think there's evidence that the reaction is simply a radical expression among true believers of the incredibulity that one's side lost the election.