You really do have to check in with the teachers from time to time. I found that one out the hard way. Some teachers do have a limited view of our kids, and if they have low expectations they will not challenge them... or when the kid does something more advanced, they pass it off as a coincidence.
I had a terrible experience with a special day class teacher many years ago, when my autistic son was in preschool. We kept getting report cards home saying how he was making such wonderful progress and she was so happy to have him in her program, but when I went for the first teacher meeting something seemed amiss... I couldn't quite put my finger on it. His behavior kept getting more and more out of control as time went on, and I made a few unannounced visits to the school. Which they hated. It turns out, my son is an imitator... there were other kids in the class who were seriously out of control, and he was mimicking their actions. They assumed he was always like that, and they did nothing to limit him or redirect him (or control the other kids) so it just kept getting worse. "They are all like that."
One day I went in to pick him up 20 minutes early, they had forgotten I was coming. He came running up to me and started signing "thirsty" right away. The teachers were talking about their weekend plans, they didn't even notice I came in. I started asking him if he had a cup to get a drink of water and the teacher finally noticed us... she came over, quite flustered, and started scolding him for getting his cup out. I told her I had said it was OK to get a drink, that he was thirsty, and she asked, "How do you know?" To which I replied, "He just told me." And she said, "He doesn't know how to say that!" And I said, "He signed it, and was saying 'tirty' which seems pretty clear to me." She looked at me like I was from Mars. She went ahead and got him a drink of water and handed it to him.. and my sweet, polite little boy said "Gank you!" (Thank you) To which she replied, "NO, Marc, this is WATER. WAAAAAAAAHHHHH -TUUUUUR." I was stunned, I told her what he had just said, and she looked at me again like I was nuts (I am but that is beside the point) and literally rolled her eyes.
The next week I gave them a list of the 200+ words that he knew and had said (not just echolalia) over the past few months, and she said we were imagining it, that he was not talking at all. Wishful thinking, and we should readjust our expectations. Needless to say, he was OUT OF THERE. She also told us he would never be a candidate for full inclusion in regular ed, and he has been a shining star as an inclusion student. He is in 3rd grade now and doing very well.
You really do have to keep an eye on them. I have had similar issues with my dyslexic daughter. I like what SueM said, IEP means "I Explain Periodically" to the teachers what it is they are supposed to be doing according to the plan.