A SUB-CONTRACTOR for Kowloon Wharf Cable Television Company was yesterday placed on probation for 18 months for having sex with his former teacher's 13-year-old granddaughter. Judge Caird, in the District Court, also ordered the 21-year-old defendant not to have contact with the girl, now 14. The accused, who admitted a count each of indecently assaulting and having unlawful sex with a girl under 16, undertook to work and reside as directed by the probation officer. He was ordered to pay $500. Prosecutor Anna Lai said the indecent assault took place in the afternoon of December 25, 1992, when the defendant and the girl, then 12, were left alone in his Kowloon flat. At her New Territories home on August 6, 1993, he again had sex with the girl, then aged 13.