Well, I thought I would fiddle around with this and figure it out for you, but after a couple of days of trying, I have not yet successfully enabled blogging for my teachers while disabling blogging for my students. Hmmm... Guess I'm not as smart as I thought.
I thought I could deny blogging to students and teachers and then create a "blogging-teachers" role and assign the teachers that role. No luck, I am afraid.
But I'll bet you anything that it's not at all difficult to do. I am just not seeing something obvious. (That happens to me sometimes!)
Anyone else care to explain how to do this step by step?