The Coal Hill defenders were a group of 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. The Doctor selected them because two of them were alien acquaintances of his and an additional three human students had impressed him.
The tear the group were tasked with monitoring 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. He felt the students were quite capable of dealing with what came through the tears, citing April's altruism, Tanya's intelligence, Ram's passion, and Charlie and Quill's alien knowledge. (TV: For Tonight We Might Die)