It is now 20 years since Alan Bennett’s The History Boys had its premiere and it is still giving a masterclass in drama. To mark the milestone anniversary, the play has been revived by director Sean Linnen and is at the Theatre Royal, Nottingham, until Saturday. And although set in a 1980s Sheffield comprehensive the themes and subjects it tackles are still hugely relevant today. Eight unruly teenagers have received the best A-level results their school has ever received and have returned to the classroom in a bid to pass exams for Oxford and Cambridge. They are clearly bright – easily quoting…