What is the ruling on using a hair mannequin for training and teaching hairstyling, given the possibility of covering its facial features?

There is no objection to using a "hair mannequin" because it is merely a head without a body, and it does not fall under the category of forbidden images or prohibited statues. An image, if its head is severed, or if a part is removed from it without which life cannot persist, is no longer forbidden. The criterion for forbidden images is the complete image into which its creator can blow a soul. Thus, if the image is only of a face, or a hand, or a leg, it does not fall under the prohibition. It is preferable to efface or cover the facial features.