Here's why:
* Active voice means the subject of the sentence performs the action. In this case, "Mrs. Walker" is the subject and she is performing the action of "telling".
Let's contrast this with passive voice:
* Passive voice means the subject is acted upon. We could rewrite the sentence in passive voice like this: "Scary stories were told to the class by Mrs. Walker". Here, the subject is "stories" and they are being acted upon by "Mrs. Walker".