Orange Tree theatre, LondonBased on real events, Pamela Carter's drama follows British schoolchildren on a walking tour in prewar Nazi Germany What started out as a bit of fun is beginning to turn ugly. The jokes are waning. The cold is starting to bite. It's 1936 and 27 boys from a school in south London are on a walking tour of the Black Forest in Nazi Germany. By the end of Pamela Carter's new play, which is insp…