The nazi stuff was very well written. With the gentle threat getting stronger, and the final stand the children make when the nazis are chasing the Jewish man. And the distinction between the Germans and the nazis, plus all the stuff with Maria Marani and her father. Very good, and very impressive for a children's book at the time, when they didn't know which way everything would go.
I understand the issue the Chalet School had with not wanting the Peace league to fall into the nazis hands. However, I maintain that it is totally silly that they're going on a picnic and a spy happens to see them, sees a piece of paper in one of their pockets and immediately jumps to the conclusion that the schoolchildren have an important dokkerment that they're going to be hiding. How many school children carry important enough stuff for a spy when they're going on a picnic. Maybe if Frau Whatever and Herman had come into the school at some point and seen it and determined to get it?
But then, on the basis that the nazis weren't exactly known for their justice system, I suspect that Madge, and probably Jem and possibly the children might well have found themselves on the way to a concentration camp after the others were found to have escaped. Or the mob who attacked the Jewish man, obviously knew Joey because she addressed them by name, so would have immediately gone round to Madge's house or the Chalet School and attacked that.
But no, the group have escaped, so the rest of the school is allowed to leave calmly and carefully, with apparently no risk.
However the School is considered so dangerous they send a school girl spy to check it over when it restarts...