My understanding as well is that it's not imperative (unless it's in your contract) but highly encouraged. It's not just for the benefit of the publisher, but yours too because of the sales you can make.
I as well would be incredibly nervous to do one - I'm definitely not the 'teacher type' and would have no idea where to begin creating a lesson plan, or how to deliver it in front of an audience (complete with stage fright and all).
One thing that I've noticed some do (Tara Lazar for example) is that if they want to do the school visits but can't/don't want to travel there, they set up Skype visits with classrooms. She likely arranges with that school ahead of time some sort of an order form that the kids can take home to their parents so that if they want to buy a copy of her book, a bulk order arrives directly to the school before the Skype visit takes place.
I don't think there's a hard and fast rule on anything - if you don't want to do them but can be creative about it in other ways to get your book out there, at the end of the day all that really matters is that your book is selling well.