I'm a former teacher, high school and college level. And I've been around my share of ultra-lib teachers. I really don't think they care where Santorum sends his own kids for an education.
Now, if Santorum came out strongly for school vouchers, which I think he would do, that would be an issue. But it would be a second-tier issue. The union leadership's heads would explode (as you said), but not so for the rank-and-file.
I really think the average teacher is more worried about things that have an immediate impact on them. Things like the price of gasoline, taxes, and why their college-graduate children cannot find work.
I hope you are right.
I am surrounded by them, and while some of them fall into the type that you describe, there are even more who are very obedient to the union talking points.