My dh is in IS as well (works with the IT dept all the time), he's a manager of a team in IS, so I am well aware of the nightly calls. We have them often in our household as well. They have disrupted many dinners, etc... However my dh and I both agree that his salary reflects all those on call nights, it's why they get paid what they do. It's part of the job description.
However, I think you are forgetting that teachers have to deal with similar things on a daily basis. The things that are in their job description. If little Johnny is having trouble, the teacher is most likely contacting the parents at night to take care of that situation (because that is when the parents are available). They get phone calls and emails from parents at all hours on issues that need to be taken care of, and they take care of them. Teachers have meetings galore after school (and often at night) that they are required to attend, IEP meetings for special Ed children (which alone require LOTS of outside time just to prepare for the meeting), meetings on the latest teaching style, ect... What I am trying to say is that teachers already do a lot outside of work that most people don't even realize. Preparing the lesson plans alone takes a lot of work, and there isn't enough time during the day to get those plans finished. They take that work home and continue it later. They already work well beyond the 9-5. It's all relative.
What is being talked about here is a social event. I'm sorry, but I don't see how someone can EXPECT a teacher to come to a social event just because it is at their school. I don't know of any profession where people are expected to come to social events, unless they are either getting paid, or it's somehow beneficial to that person's career. It's not a social event otherwise. Teachers already do a lot for their schools, they can't do everything. If they went to all the sporting events, school dances, you name it, they would never see their own family!! I understand that you think you are only asking them to come to one once in a while, but they also want to spend some time with their families once in a while too! Just because you don't see them at a school dance does not mean they didn't attend another event at another time, or didn't already spend a ton of extra time working on school stuff that week. Follow a teacher around for a week and I'm pretty sure your opinion would change.