Looking for an '70-'80's movie with school kids on holliday

[Telecolor 3007]

Looking for an '70-'80's movie with school kids on holliday on a camp. The only thing that I rember about the movie:
1) They where going to the camp with the train!;
2) At the train station, before the departure, boys are throwing with water filled ballons;
3) On the train, one of the kids hids a chewing-gum on his mouth in order to pretend that he haves dental problems.
Does any one knows the name of the movie?