Job satisfaction, teacher life-cycle, age, gender, ethnicity, education level, and internal belief systems are some of the variables researchers have studied regarding teacher retention and attrition. These variables may be categorized as external and internal variables.

Justice, Greiner, and Anderson (2003) state 75% of the demand for newly hired teachers is due to teacher attrition.

External:

Some of the major external variables include school facilities, violence on the campus, and teacher interactions: student-teacher, teacher-teacher, teacher-administrator, and teacher-parent

Administrative Support and Leadership Resources
Cooperative Environment and Collegiality School characteristics (school size, district size, location, student enrollment)
Parental Support
Student Behavior and School Atmosphere
Credentialing Requirements
Professional Development
Autonomy in the Classroom
Autonomy in the School
Compensation (Kelly, 2004)
Mentoring Programs
Out-of-field teaching assignments
Change in family situation (Kirby & Grissmer, 1993)
Internal:

Teacher characteristics include:

Age. According to Guarino et al (2004), a key factor to teacher turnover is age. Young and old teachers leave teaching more than middle aged teachers, leading to a U-shape.
Ethnicity
According to Guarino et al. state minority teachers remain in teaching longer than Caucasians.
Gender
Females remained teaching longer than males (Guarino et al, 2004).
Socioeconomic status
Education level
Teachers with higher measured abilities leave teaching more than peers (Guarino et al., 2004).
Credential status