HONG KONG - Hong Kong Special Administrative Region Chief Executive Carrie Lam on Tuesday expressed concern over the involvement of students in illegal activities amid social unrest that has lasted for six months.

At a press briefing in the morning, Lam said she had asked schools to prevent their students from participating in illegal activities and follow the notice sent by the Education Bureau at the end of last month.

The police have arrested 6,022 people for offenses including unlawful assembly, participation in riot and possession of offensive weapons, among others, since June 9. Among them, 2,393 people or about 40 percent are students, and about 300 high schools in Hong Kong were involved.

Of the 12 people arrested on Monday morning, six were students and one was a teacher. Police seized flammable liquid and glass bottles, which could be used to make petrol bombs, and instruments for road blockage and vandalism, including nails, electric drills, homemade barricades for puncturing tires.

Lam said she had asked the secretary for education to seriously follow up the case of the teacher under arrest.

Two home-made remote-controlled bombs were found inside the campus of Wah Yan College, Wan Chai on Monday and were later defused by the police. The bombs were fully functional and may cause multiple casualties, according to the police.