worker in any other profession, so that's not surprising--we see errors everywhere. So it would be pretty high expectations to expect them to be experts at understanding kids (where most of their classes are) AND English. That'd be like asking someone in any profession to also be an excellent writer.
Now that being said, I'm a big grammar guy and think EVERYONE could do better at that, and we should probably require more in college to learn how to write well—teachers included as well as everyone else.