The magistrates' sentencing guidelines state that they should
consider a custodial sentence for a breath reading above 115. However, they're never obliged to pass a custodial sentence and the chances of them doing so here are very low. (Teacher's don't have lengthy criminal records or they wouldn't be teaching. Additionally, the prisons are full and courts aren't going to rush headlong into adding to their population. They also hesitate to take actions which would ruin the career of someone in the 'caring professions').
He's probably looking at a 3 year driving ban, a fairly hefty fine (+ costs + victim compensation scheme payment), and a rehabilitation course. The court might also impose impose a community penalty or a suspended prison sentence but both of these options are probably unlikely.
See page 70, here, for further guidance:
http://www.jsboard.co.uk/downloads/acbb/sectio n2a.pdf
Chris