The US first lady did not explicitly criticise Cambodia's human rights record or its government, which has been ruled for 30 years by authoritarian strongman Hun Sen, the prime minister.
But she sent a pointed message, and one that she delivered seated beside Hun Sen's wife, Bun Rany.
Mrs Obama is on a five-day trip to Asia to promote the US-led education initiative "Let Girls Learn," which she and the president announced earlier this month.
"Let Girls Learn" is starting in 11 countries, including Cambodia. Mrs Obama's visit follows a three-day trip to Japan, which is helping to fund the project.
The trip has given the first lady, who is travelling without her husband, a chance to soak up some of Asia's rich culture.
In Japan, she visited Buddhist and Shinto shrines, and in Cambodia she travelled to Siem Reap, home to the famed Angkor Wat temple complex.
"You are role models to the world," said Mrs Obama, seated on a wooden school chair beside the students and Cambodia's first lady. Education brings empowerment, she added, and urged the teens to "finish your education and then follow your dreams."
"Not just here in Cambodia, not just here in Siem Reap, but in the world. I hope that you all will feel empowered to do that."
As Mrs Obama spoke, Bun Rany smiled and made no comment. However, she expressed Cambodia's "full support" for the education initiative, and said the government was giving priority to female students for state scholarships.
Mrs Obama's trip marks the first by a sitting American first lady to Cambodia. Barack Obama became the first US president to visit Cambodia in 2012.
