I think you've had some excellent advice on this thread, but I agree with Queen. I think this might have gone further.
The thing is, you have had an awful shock today and your feelings are likely to be all over the place. Part of you will be desperate to retain some normality because of the timing, it being Christmas.
I think the reasons he's reacted this way are quite simple. He wants to 'hide away' to give himself some thinking time and the defensiveness is because you have taken his Christmas toy away, by finding out. He is unlikely to be sorry for his actions, but he's likely to be sorry he got found out and sorry that it might have to end.
It's going to take some getting your head around that unlike other problems you've faced in your marriage, this can't be sorted before Christmas. If you can't face asking him to leave, then think about telling him quite calmly that you will be seeing a solicitor, because you have checked and discovered that you have grounds for divorce (and you do btw.)
I would explain that he is staying put only because it's Christmas, but that unless he can prove to you that he is genuinely sorry and is willing to explore within himself, why he has done this - as far as you're concerned your marriage is over.
In other words, put the onus on him to sort this mess out. Carry on with what you need to do, talk to people you trust if you'd find it helpful and until he 'gets' this, treat him with polite indifference.
Oh and I'd also tell him to look for another job, as a condition of staying in his marriage. Unlike other industries, at the moment a good teacher can always get a job.