By Abigail O’Leary Gunmen attacked a school in Nigeria’s northwest region Thursday and abducted at least 287 students, the headteacher told authorities, marking the second mass abduction in the West African nation in less than a week. Abductions of students from schools in northern Nigeria are common and have become a source of concern since 2014 when Islamic extremists kidnapped over 200 schoolgirls in Borno state’s Chibok village. In recent years, the abductions have been concentrated in northwestern and central regions, where dozens of armed groups often target villagers and travelers for h…