Spy classroom short story collection, vol. 2 (light novel): the spy teacher who loved me - takemachi

Before she dies, Hearth, the leader of the legendary spy team Inferno, bequeaths a key of unknown purpose to Thea, a young girl who saved her from the brink of despair.

Years later, Thea finally discovers a box that the key can open.

Inside, she finds Hearths last testament--but its missing a word.

Soon, misunderstanding begets misunderstanding as everyone in Lamplight gets sucked into uncovering the mystery of Hearths note!.