Must admit to having no idea what type of shoe a plimsole is ( I m guessing its a type of sandshoe ).
Anyway... in the middle of winter in colder parts of the country ie Central Otago , many children will wear sandshoes / sneakers or maybe simply the shoes they wore to school. In warmer parts of the country with no hoar - frosts , the children will most likely be running around in barefeet. You'll get used to the barefoot scene. My children have a PE uniform but its only worn on major sports days.
At high school you will find "most " kids wearing the appropriate footwear for the particular sport they are involved in, however its certainly not unusual to see a game of lunchtime rugby/ tennis being played bootless/ shoeless.
Rucksacks backpacks are used by the older children too.