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How to change in a school locker room
Wear clothes that are easy to take off and put on.
On the days that you have gym class, make sure you wear stuff that's easy to get in and out of. Stick to shoes that are easy to lace or slip on, and pants and a top that are easy to take off on gym days.
You'll be able to get out of these clothes-and into your gym clothes-quickly. For example, skirts with elastic bands, comfortable jeans, pullover shirts, and slip-on flats are great options for gym days.