Technically only one ghost has ever been shown to be able to speak being the girl with no eyes in ch3. Even in this chapter the ghost never spoke a word even when spoken to, so it might be normal for ghosts to be unable to speak. The more noteable thing is just how different Aizawa-san is shown from other ghosts. Not only does she look as she did in life with no grotesque features, but she has unique abilities other ghosts don't seemingly have like telekinesis and possession. She also is the only ghost bound to a location noteably with a pentagram which is a common way of depicting the occult in Japan
She also isn't able to touch Kurotori-san which we know all other ghosts are capable of which is even weirder when you consider the pentagram that binds her was moved from below her desk, to the wall of the school, and finally to the back of Kurotori-san's neck making her currently bound to Kurotori-san herself; yet, can only touch her indirectly via possessing someone...