"The duck was excellent in swimming, in fact better than his instructor, but he made only passing grades in flying and was very poor in running. Since he was slow in running, he had to stay after school and also drop swimming in order to practice running. This was kept up until his web feet were badly worn and he was only average in swimming. But average was acceptable in school, so nobody worried about that - except the duck." This happens too often to some children. Their passion for a topic is killed by the need for schools to keep everyone learning at the same pace in the same subject areas.
The tragedy of the duck's loss of excellence bears itself in her feet. The pursuit of a broad education - which is a positive - trumps one duck's natural talent to swim. The author's show the imbalance in this "heroic" school.