Don't be too hard on yourself. I was going to comment before that after 12 days (even though it feels like a long time) it sounds like they have come so far. It's normal to have some set backs and bad days. Going back to school after a long break may have just been a little too stressful and confusing for them.
When my daughter had some bad set backs and I felt like I couldn't trust to take her out with her underwear, and it just stressed me out and made me angry when she had an accident, I put a pull up on over her underwear. It seems like the same thing, but maybe to them they'll realize it is still their underwear and they're not supposed to get them wet. If they do, at least the clean up is easier and less stress on you. And at least with a pull up vs a diaper, they can pull them up and down themselves, so they can choose to still go to the bathroom on their own at school. Just a thought. Potty training sucks. My daughter still has accidents sometimes - a couple this week, actually, and she's been trained for a year.