U.S. President Barack Obama has told America's schoolchildren that nothing will have greater impact on their success in life than education.

Mr. Obama delivered a "Back to School" speech at a school in the northeastern U.S. city of Philadelphia, Pennsylvania Tuesday.  He told the students that they have an obligation to themselves to get the best education possible, and that the country has an obligation to make sure that the best education is available.

The president acknowledged that the economic downturn has forced some students to take more responsibility within their families, helping out with child care or getting a part-time job.  Mr. Obama said the strain should not force students to lower their goals.  He told them to "dream big" and stay focused on education.

Mr. Obama also announced the second Commencement Challenge, a contest among schools in which the winner will have the president speak at its graduation ceremony.

Mr. Obama has long pushed for a better-educated work force, which he says will help the U.S. compete globally.

He delivered his first back-to-school speech last year at a high school near Washington.  Some conservatives protested that speech, claiming he was trying to indoctrinate their children or push a domestic agenda.