KADUNA, Nigeria—The kidnap for ransom business is booming across northern Nigeria, and schoolchildren are its hottest commodity.

Just before midnight on March 11 gunmen barged into a school around 300 yards from a military training college in Kaduna state and seized dozens of students from their dormitories. It took less than 12 hours for the captors to issue a now familiar demand, through a grainy video posted on Facebook.

“They want 500 million Naira,” said one of the terrified hostages from the Federal College of Forestry, sitting shirtless in a forest clearing, a sum equal to around $1 million. Masked men wielding Kalashnikovs paced among the 39 students—mostly young women—then began to hit them with bullwhips.

“Our life is in danger,” a woman screamed. “Just give them what they want.”

On March 13, the Nigerian army foiled an attempt to kidnap 300 more students at a boarding school less than 50 miles away. The following day, children were among a group of 11 people abducted from the town of Suleja, in Nigeria’s Niger state.