I just think it's a tiny bit odd for parents to "pop in" to see what their child is up to, unannounced. Of course, I'm not a parent yet, and when I am, I may have a very different perception, but, I do think it's a little
bit strange.
BUT ... in defense of parents, school has changed SO MUCH since most of us were there. It's a lot more fun and interesting and innovated. There are class pets, centers, all kinds of work displayed, books, and many other things that kids can take ownership over. For parents, it can be really exciting to come in and see all of this in motion.
I think that parents
who ask to come in and observe are more often than not just plain interested and not being nosy or underhanded.
I do think though that a parent requsting to come in to "observe the class" is overstepping a bit. Maybe it's just the wording.