(CNN) When students across the US walk out of class for 17 minutes today, many of them will be wearing orange.

It's more than just a simple choice in clothing -- it's meant to send a powerful message.

The color choice dates back to at least 2013 when 15-year-old Hadiya Pendleton was shot and killed in Chicago. Hadiya was an honor student who was killed in a case of mistaken identity -- a week after she performed at one of the events surrounding President Obama's second inauguration.

Her friends picked the color orange to honor her and to call for gun reform.

Why? It's what hunters wear to protect themselves -- from other hunters.

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