Coal Hill defenders

The Coal Hill defenders were a group of four students and a teacher from Coal Hill Academy that were assembled by the Twelfth Doctor to protect the school from any alien threats that came through a tear in space-time. (TV: For Tonight We Might Die) A few weeks after the group was formed, though, April MacLean, Tanya Adeola, Ram Singh, Charlie, and Miss Quill regarded themselves as the only members of the group, with Matteusz, the fifth student, not being a member. (PROSE: What She Does Next Will Astound You)

The tear had been opened by an excess in artron energy around the school, and after an attack from Corakinus and the Shadow Kin the Doctor was able to close the tear. However, he warned that he couldn't fully close the tear, and that Coal Hill would continue to transmit like a beacon across the universe to any nearby aliens who wished to use the tear to enter to Earth. (TV: For Tonight We Might Die)