The former 'Coronation Street' actress will lead the cast of the new series, which will air on BBC Three later this year.
The drama is set in Coal Hill school in contemporary London, and will see Katherine joined by four newcomers - Greg Austin, Sophie Hopkins, Fady Elsayed and Vivian Oparah - who will play sixth-formers.
The show's creator, Patrick Ness, said it was a coup to get Katherine, who recently starred in hit BBC spy drama, 'The Night Manager'.
"How lucky we are to get Katherine Kelly!" he said in a statement.
"She's been stunning in 'Happy Valley', 'The Night Manager' and 'Mr Selfridge', just wait until you see her here."
'Doctor Who' boss, Stephen Moffatt, added of the show: "There's nothing more exciting than meeting stars that nobody's heard of yet. We had the read-through of the first few episodes last week, and there was a whole row of them.
"Coal Hill School has been part of 'Doctor Who' since the very first shoot in 1963, but this new show is anything but history.
"'Class' is dark and sexy and right now. I've always wondered if there could be a British Buffy - it's taken the brilliant Patrick Ness to figure out how to make it happen."
'Class' follows previous 'Doctor Who' spin-offs 'Torchwood' and 'The Sarah Jane Adventures'.
It is also the latest exciting commission for BBC Three following its move online, having recently enjoyed success with mystery drama series 'Thirteen'.