GAO, Mali (AP) -- 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, after tens of thousands of people fled the fighting and the extremists' strict Islamic rule.

However, other pupils left Gao not with their families but with the Islamic fighters when they retreated, say human rights activists and local officials.

What happened in 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.

Abdourhamane Maiga, assistant director of the Adadatou Alislamiatou madrassa, recalls one student who dropped out of school after being asked to repeat a grade.

The next time Maiga saw the pupil, he was wielding a firearm with the Islamic fighters at their police headquarters downtown.

"They didn't come here to practice Islam," he says of the extremists. "The prophet never would have accepted a child of 10 years old waging jihad and taking up arms."