HONG KONG, Aug 27 (Reuters) - A Hong Kong court on Friday approved bail for a student leader who is among four charged with "advocating terrorism" after their union passed a motion mourning the death of a 50-year-old who stabbed a police officer before killing himself. As part of the bail conditions, High Court Judge Esther Toh ordered Yung Chung-hei, 19, to pay HK$50,000 ($6,420) and his mother and uncle an additional HK$50,000 each for surety, reject media interviews, report to police four times a week and surrender his travel documents. Yung must obey a curfew, not contact foreign officials, not join any student organisations at an executive level, and not "directly or indirectly" do or say anything on public platforms that might contravene a national security law imposed by Beijing last year. Hong Kong laws prohibit publication of the content of bail hearings. Under the security law, a judge must believe a defendant will not continue to endanger national security if granted bail, a bar which has resulted in most defendants being kept in custody. Yung was initially granted bail by Magistrate Peter Law at West Kowloon Court last week, but prosecutors immediately appealed the decision. The other three students, Kinson Cheung King-sang, 19, Kwok Wing-ho, 20, Chris Shing-hang Todorovski, 18, were all denied bail and are in custody. The case was adjourned to Sept. 14. The police officer was stabbed from behind on July 1 while trying to prevent demonstrations on the anniversary of the city"s return to Chinese rule in 1997. The man then stabbed himself in the chest and died after being taken to a hospital. The police officer, 28, suffered a punctured lung, but survived what Secretary for Security Chris Tang described as a terrorist act by a "lone wolf". Leaders of the union later apologised, retracted the motion and resigned from their posts. The union"s campus office has since been raided by national security police. The university has severed ties with the union and banned students who signed the motion from entering its premises. ($1 = 7.7876 Hong Kong dollars)