At least thousands of secondary school students are set to boycott classes every Monday starting from September 2 if the Hong Kong government refuses to withdraw the now-shelved extradition bill, according to an online opinion poll.The survey was conducted by two student concern bodies and pro-democracy group Demosisto, co-founded by political activist Joshua Wong Chi-fung. The group earlier called for a general boycott, following two months of civil unrest sparked by the hated legislation,…