YAOUNDÉ, Cameroon—Walking from classroom to classroom at the Ahl ul Bayt Linguistic Center, which teaches Arabic and Islam, director Ahmed Tijani pointed at his students, a mixture of teens and young adults.

“This one is Shiite, these ones are also Shiite,” he said. “And these ones, they are still Sunni.”

Mr. Tijani, whose office in the Cameroonian...