Here's why:
* Predicate adjectives modify the subject of a sentence and follow a linking verb (like "is," "are," "was," "were," "seems," "becomes," etc.). They describe the subject.
* This sentence uses an action verb: "teaches". The verb "teaches" describes what Mary does, not what she is.
Let's look at an example with a predicate adjective:
* "Your sister Mary is a talented teacher."
* "Talented" is the predicate adjective modifying the subject "Mary". It describes her quality.
To make your sentence include a predicate adjective, you could rephrase it like this:
* "Your sister Mary is a math and physical education teacher at high school."
* "Teacher" is the predicate adjective describing Mary.