Albus did block her, grabbing her wrist as she came at him and twisting her arm around. He kept an eye out for her other arm to come swinging at him, unsure if she was going to come at him from another direction as well.
With her facing away from him, she wouldn't be able to see his foot come out to kick at the back of her knees to knock her off balance. He had told her right up front that he would return to her what she did to him, and that was even before she had agreed to teach him to spar.