If the:
Student isn't a troublemaker.
Student isn't real outgoing.
Student gives an average performance.
What are other possible reasons for it?
Teacher thinks student is hot.
Weird and disturbing but probably true
teacher was bribed by student.
student is a kiss-up.
teacher related to student.
teacher is a weirdo.
teacher same ethnicity as student.
The teacher sees potential in this particular student