Re: I have no problem with teaching ID in a school but...
Why is it okay for a science teacher, in response to expressed interest from the students, to devote a class or two to string theory, but not to ID?
Well, I think motive goes a long way. ID and creationism have been developed - openly, by their proponents - as a means to combat secular public education by creating a "wedge" issue they can use to insert official recognition of the existence of the Christian God in American public schools.
String theory is just a way to reconcile the untenable number of "elementary" particles in the universe. While it's essentially evidence-free at this point, it's also ideology-free; it's not a political wedge issue. It's basically Trivial Pursuit for eggheads.
Honestly I don't think creationism is completely out of place in the classroom; I don't think it would be inappropriate for a teacher, in response to a question about it, to inform students about what creationists contend and what evidence contradicts their view. I think, largely, teachers should concern themselves with true things; and to say that there are creationists, and that they believe such-and-such, are true things. Similarly, it's true that there is string theory, and it's proponents contend so-and-so, so I don't see that informing students of that is any big deal.