For many 12th graders, the closure of Nigeria´s public schools to combat the spread of COVID-19 presents a particular problem: How to prepare for crucial, final exams? Basirat Olamide Ajayi, a math teacher in Lagos, Nigeria´s biggest city, came up with a solution. She began offering free mathematics classes online via Twitter, WhatsApp and Instagram. And now, after almost six months, more than 1,800 students at various levels are taking her classes — across Nigeria and even internationally. Students watch her short math videos — no more than 5 minutes long — and respond to her questions. She will send them homework, and occasional assignments. And she grades them. “Sometimes, I stay awake till 2 a.m. going through their assignments!” she said. “COVID is here with both negative and positive impacts. The positive impact is that we can use technology to teach our students, which I am very, very happy about,” she said. When Ajayi, 36, started her online classes, she solved math problems on camera on white sheets of paper. Then a parent saw how she was conducting the class and donated a whiteboard. Her free classes are attracting students from all over Nigeria, and now students abroad are joining. A recent request came from Canada. Ajayi says she is beginning to see herself as a global teacher. “The online teaching has made me feel that I can actually teach the whole world mathematics,” she said. “On Twitter people see me all over the world, not only in Lagos, not only in Nigeria. They see me all over the world and that is enough to give me innermost joy.” But not all students in Nigeria have easy access to her lessons. “Some of them don´t even have data to access the class, and that is not giving me joy at all, as a teacher that wants students to be online,” she said. Ajayi said she pays for data for some of the students from her own pocket to allow them to be online. Some students don´t even have phones; Ajayi encourages parents to share their phones. Fortune Declan, 17, said Ajayi has made it easier for him to grasp mathematics. “Originally when I started learning differentiation on my own it was kind of twitchy,” he said. “But when I joined the online maths platform, I started slow at first, but with the way my maths teacher was teaching, holding the sessions, I started learning differentiation rapidly.” Her dedication is noteworthy, said Adedoyin Adesina, chairman of the Lagos arm of the Nigerian Union of Teachers.