Washington  - US President Barack Obama took to the airwaves Tuesday to encourage the nation's students to focus on their studies, but many children did not hear the message as some schools chose to boycott the speech that critics had dubbed political indoctrination.

The speech at Wakefield High School in Arlington, Virginia, had unleashed a political firestorm on what was the first day of classes for many of the nation's school children.

The White House released the text of the speech in advance to turn down some of the controversy. Many school districts in conservative areas chose not to show their students the speech because of concerns the Democratic president might make political statements. Others decided to allow each school or each teacher to decide whether to air the speech.

In the live address, Obama detailed his own experiences in school and the difficulties in his life being raised by a single mother. He urged students to fulfil their responsibilities and said they should work hard no matter what challenges they face.

"We need every single one of you to develop your talents, skills and intellect so you can help solve our most difficult problems. If you don't do that ­ if you quit on school ­ you're not just quitting on yourself, youre quitting on your country," Obama said.

The White House pointed out that past presidents, including Republicans George HW Bush and Ronald Reagan, had also spoken directly to US students.

But many Republicans had spoken against the idea in the days leading up to the speech.

Florida Republican Party chairman Jim Greer last week called the speech a bid to indoctrinate children in Obama's "socialist ideology."

"The idea that school children across our nation will be forced to watch the president justify his plans ... is not only infuriating, but goes against beliefs of the majority of Americans, while bypassing American parents through an invasive abuse of power," he said in a statement.

Greer backed down somewhat after reading the speech Monday, saying it was not political in tone and that his own kids would be watching. But he said Obama should never have given a speech to school children in the first place.

Others, including former first lady Laura Bush, defended the idea as a nonpartisan attempt to inspire.

"I think there is a place for the president ... to talk to schoolchildren and encourage" them, she told broadcaster CNN in an interview.

Part of the firestorm centred on a lesson plan released to complement the speech that had asked students to write about how they could help the president. The White House revised the assignment suggestion after the controversy broke out, instead calling on students to write about achieving their own goals.  dpa