Scottish actor David Tennant is heading back to school - he has signed on to play a villain in the sequel to St. Trinians.The star has landed his first major role since quitting as the beloved Time Lord in iconic sci-fi show Dr Who last year.He will play evil baddie Pomfrey in the latest installment of the franchise, set in a chaotic all-girls school in Britain.He will be joined by Colin Firth and Rupert Everett , who both starred in the 2007 movie - alongside Girls Aloud star Sarah Harding The 2007 remake, which kickstarted Bond girl Gemma Arterton 's career, was a surprise hit, raking in $22 million (£15 million) at the British box office.