Obama's planned address to students has prompted a surprising push-back from some quarters over what the White House sees as an important but innocuous topic.
Some conservative critics say Obama is trying to promote a political agenda and overstepping his bounds, taking the federal government too far into public school business.
Republican Gov. Tim Pawlenty of Minnesota, a potential presidential contender in 2012, said Obama's speech is "uninvited" and that the president's move raises questions of content and motive.
Many school districts have decided not to show Obama's speech, to be delivered at 12 noon EDT Tuesday, partly in response to concerns from parents.
Randi Weingarten, president of the American Federation of Teachers, on Friday defended Obama's plan to address students.
"The bottom line is we need the president of the United States of America to use his bully pulpit to talk to kids about the importance of education and to help inspire kids," she said on "The John Gambling Show" on radio station 710-AM in New York.
Gibbs said former Republican presidents Ronald Reagan and George H.W. Bush delivered similar speeches to students. He said Obama's speech will not be partisan but rather a chance for children to get "a little encouragement as they start the school year."
The White House spokesman said he couldn't speak to the motivations of some school districts.
"Look, there are some school districts that won't let you read 'Huckleberry Finn,' " Gibbs said.
He said the administration understands that some districts have logistical concerns with the timing of Obama's speech.
The White House plans to release the speech online Monday so parents can read it. Obama will deliver the speech at Wakefield High School in Arlington, Va.
Associated Press writer Marcus Franklin in New York contributed to this report.