Henderson's Boys: Secret Army, Paperback/Robert Muchamore

Britain, 1941. The government is building a secret army of intelligence agents to work undercover, including a group of kids cut adrift by the war and who, for official purposes, do not exist. They are parachuted into unknown territory on a daredevil exercise in which they will have to outsmart a bunch of adults and fight for their lives. The third in the pre-CHERUB series, not suitable for younger readers.