"Miss Dear was supplanted by a hard case called Miss Turnbull" explain: be supplanted by, a a hard case.
hard case = someone who is difficult to deal with
"there was nothing to do except duck" explain: to duck
to duck = to avoid answering a question or dealing with a situation
"he examines the shifting friendships and allegiances between the boys and their teachers" explain: allegiance
allegiance: to support for a leader, country, group, or belief: