HONG KONG—Nine people, including six secondary school students, were arrested in Hong Kong on July 6 for allegedly plotting to set off homemade bombs in courts, tunnels, and trash cans as political tensions rise in the city where China is tightening its grip. Police said the nine were detained on suspicion of engaging in terrorist activity under a harsh national security law that Beijing imposed a year ago as part of a crackdown on dissent in the former British colony that has long enjoyed freedoms not seen on the Chinese mainland. Hong Kong authorities have used the law, enacted in response to anti-government protests that rocked the city in 2019, to arrest many of the city’s prominent activists. Others have fled abroad as a result. If the allegations are true, the group appears to represent a more radical fringe of the protest movement, which has demanded broader democratic freedoms for …