Hi Tinky. I think this is a really interesting question and quite difficult really. I'm sure it must affect a lot of middle-income families when they have a second child. I am pg with my first child, I am a teacher (university) and will probably be in the same position as you if we're lucky enough to have a second DC. I am already wondering what to do!
Everyone is different, but for me it would depend on how much I enjoyed work. If you find your job fulfilling and you would miss it, keep it, because this is what will make you happier. I think your happiness will have an impact on the rest of the family - your children will pick up on it. If you are happy to stay at home, that is the best option. I imagine that as a teacher, you would be able to find another job relatively easily (sorry if that's naive - and I guess it depends where you're living). That would be a factor for me as well, because university teaching jobs are hard to come by and I worked in crap conditions for years to finally get my current job at the age of 30. For that reason I'd be loathe to give it up, even if I only worked 1-2 days a week.
If you are not sure either way, is there any way you can take a career break, or at least the full year mat leave to get a feel for what it's like to be a SAHM?
Also, don't worry about committing yourself forever, because you can always change your mind.
Good luck deciding - it is such a big decision, and very individual, so listen to others' advice with an open mind by all means, but ultimately go with your gut feeling. x