The radical Islamic fighters showed up at Mohamed Salia's Quranic school, armed with weapons and demanding to address his students.
The leader, named Hamadi, entered one of the classrooms, took a piece of chalk and scrawled his message on the blackboard.
"How to wage holy war," he wrote in Arabic. "How to terrorize the enemy in combat," the lesson plan continued.
Then his mobile phone rang, and he stepped away to answer. Salia urged his students to pose some questions of their own when he returned: Where had he come from and what did he want with a bunch of young people?
Hamadi told the students that people didn't ask questions like that where he was from. Islam knows no nationality, he replied and then left - and did not return before the French-led military operation ousted him and his fighters from power last month.
"I told my students to be careful: that these men may be well-versed in the Quran but their Islamic point of view is not the same as ours," the teacher recalled.
Nearly a month after the al-Qaida-linked militants were driven out of Gao and into the surrounding villages, students are now returning to the city's Quranic schools.
Many classrooms, though, are still half full, as tens of thousands of people fled the fighting and strict Islamic rule the extremists.
However, other pupils left Gao not with their families but with the Islamic fighters when they retreated, say human rights activists and local officials.
The experience of the Gao schools illustrates how the extremists used madrassas in northern Mali to indoctrinate young people and to recruit child soldiers.
The Islamic radicals attacked Gao several times this week, their second assault on the strategic city since they retreated in the face of French and Malian military, and their young recruits appear to be part of the strategy of MUJAO, or Movement for Oneness and Jihad in West Africa.
"MUJAO took many of the students from the Quranic schools because they speak Arabic and are easier to convert and manipulate," Gao Mayor Sadou Diallo told The Associated Press. "Between 200 and 300 children have disappeared with the jihadists."
"The schools were all complicit. They didn't have a choice - if you didn't collaborate with MUJAO you died," Diallo said.
An untold number of children are believed to have been killed in the January fighting in central Mali, he said, and when jihadist strongholds were bombed in Gao during the military intervention last month.
Dozens of child soldiers were believed to be living in a government customs building that was later bombed during the military offensive, residents say. The Islamic fighters took away their wounded before it could be determined how many casualties there were at the site.
The rubble of the building is littered with tiny children's shoes, and notebooks and pieces of wood on which the children copied Quranic verses. The children's writing in pen on notebook paper depicts verses seeking protection from evil.