HONG KONG — The 15-year-old student woke Monday morning, donned his school uniform of white shirt and blue pants, and left the Hong Kong apartment he shares with his parents and grandparents.

On the first day of school Monday, secondary students in Hong Kong supplemented their formal white uniforms with gas masks, goggles and hard hats.

Hong Kong authorities on Wednesday banned school students from singing of “Glory to Hong Kong,” the unofficial anthem of the pro-democracy protest movement, just hours after Beijing set up its ...