A friend and I have been going round and round about this.
He says to listen to what the President says and then decide.
I say that context is everything, and that when you get your union acolytes to compel kids to listen to you, and then give the students assignments like "write a letter explaining how you'll help Barack Obama", when you see how a cult of personality (see below) is built around this President, and you recognize his style of politics--it's not unreasonable to be suspicious of his motives. I'm sure his talk will be fairly benign, but maybe it'll be more benign now that he realizes that people will scrutinize what he says. Heck, the White House has already removed the "help Barack Obama" part from the lesson plans due to outrage, so someone there is listening.
Want to see what worries me about this type of activity from this particular man? Watch this video, especially near the end--even though you'll want that 4+ minutes of your life back--and this one, which shows how easy it is to indoctrinate students. In both videos, people are pledging allegiance to President Obama himself, not to the nation or its ideals.
It's a bit too Dear Leader-ish for my taste.
My suggestion: give the speech during prime time, advertise the heck out of it, and in those advertisements, ask parents to watch it with their children. If you're going to talk about the importance of hard work and staying in school, that should be the target audience. If he did that, heck, I'd probably watch it with my son.