Tsang Yok-sing, chairman of the Democratic Alliance for the Betterment of Hong Kong (DAB), is to become a full-time politician. Mr Tsang, 50, will step down as principal of Pui Kiu Middle School to stand in next year's legislative elections. A party source said: 'He spends a lot of time on DAB affairs and finds it difficult to manage it and the school.' Mr Tsang, a teacher at the North Point school since 1969, became principal in 1985. A DAB founder, he contested the 1995 Legislative Council elections but lost to Bruce Liu Sing-lee of the Association for Democracy and People's Livelihood in the Kowloon Central constituency. Mr Tsang, currently on holiday in Canada, will announce his decision to resign at the school's graduation day on Sunday. One source expected Mr Tsang to continue to serve the school, but in a different capacity. However, Gary Cheng Kai-nam, a longtime professional and political colleague of Mr Tsang, said the school council had yet to decide whether he would be offered another post. The source said he was likely to bid for a Legco seat through direct elections. Other DAB members set to run in the elections are post-handover executive councillor Tam Yiu-chung and provisional legislature members Mr Cheng, Ip Kwok-him and Chan Yuen-han.