Speaking at the press launch of Last Christmas, the team announced that Jenna is confirmed for staying the whole of series 9.

Steven Moffat commented:

"Jenna is in all of the next series, which we’re delighted to reveal.  There was a certain amount of to-ing and fro-ing"
"I always think I’m quite good at winding up people on this but Jenna completely outplayed me when she said in an interview 'I’m not telling you if I’m staying or going because it’s just more exciting'. And it occurred to us all that we don’t start shooting until January so we could actually keep it a secret."

Jenna Coleman confirmed that Peter Capaldi helped sway her into staying:

"It’s wonderful – a whole other series of stories with the Doctor. I couldn’t walk away with the story being unresolved, there’s so much more to do and I think [Clara and the Doctor] have finally just reached a point where they really understand each other." "The arrival of the 12th Doctor has just dropped this bombshell and allowed the dynamic to totally, totally change so I think, just when Clara was feeling more comfortable in the relationship, it’s suddenly thrown something totally new up and I think the comeuppance of that and the story that we got to tell over the season has been thrilling."

Peter Capaldi also commented:

"I think Jenna’s been just fantastic and it’s been such a pleasure to work with her – and especially for me as the new person onto the show, she’s just been fantastic – and so I was so excited and delighted when she finally [agreed to stay]." 

But with an episode entitled The Magician's Apprentice, are we looking at two companions?