First ladies normally refrain from outspoken foreign policy remarks, but Mrs Obama has been a vocal campaigner for the release of more than 200 girls.
Fifty-three of the schoolgirls escaped soon after being seized in Chibok on 14 April but many more remain captive.
Islamist militant group Boko Haram claimed responsibility for the attack.
The United Nations Security Council expressed outrage over the abductions in north-eastern Borno state, and demanded the immediate release of the students.
It said it would consider “appropriate measures” against Boko Haram. The US is seeking to have UN sanctions imposed on the group.
‘Call to action’
“As the mother of two young daughters, Mrs Obama is taking up the opportunity to express outrage and heartbreak the president and she share over the kidnapping,” White House deputy spokesman Eric Schultz said.
“The first lady hopes that the courage of these young girls serves as an inspiration… and a call to action for people around the world to fight to ensure that every girl receives the education that is their birthright”, he added.
Michelle Obama has often appeared alongside her husband during the weekly address, which is broadcast on radio with a video version available online. This is the first time she will deliver the speech alone.
Earlier this week, she tweeted a picture of herself in the White House holding a sign with the message “#BringBackOurGirls”.
President Obama has described the kidnapping as “heartbreaking” and “outrageous”.
A small number of US and British experts have now started arriving in Nigeria to assist the government’s rescue efforts.
A senior US official said Washington was also considering a Nigerian request for surveillance aircraft.
But analysts have pointed out the difficulty of using aerial reconnaissance in the region’s rugged forests.
British High Commissioner Andrew Pocock told the BBC’s Today programme: “The eye in the sky, even if it were able to be focused on the spot, isn’t a panacea.
“What is a way forward is the proper use of… gathering of intelligence, which the Nigerians are attempting to do and which all the Western friends of Nigeria can help them with.”
Nigerian President Goodluck Jonathan said on Friday he believed the schoolgirls were still in his country and had not been moved to neighbouring Cameroon.
Amnesty International on Friday claimed Nigeria’s military had advance warning of the raid but failed to act.
Nigeria’s authorities say they “doubt the veracity” of the human rights group’s claim.
Boko Haram has admitted capturing the girls, saying they should not have been in school and should get married instead.
In a video, group leader Abubakar Shekau threatened to “sell” the students.
Boko Haram, whose name means “Western education is forbidden” in the Hausa language, began its insurgency in Borno state in 2009.
At least 1,200 people are estimated to have died in the violence this year alone.
Source : BBC