The board of a school district may dismiss an individual employed as a teacher, a principal, or as an assistant or associate superintendent prior to the expiration of the individual's contract for any of the following causes:
1. Immoral conduct.
2. Insubordination.
3. Conviction of a felony.
4. Conduct unbecoming the position held by the individual.
5. Failure to perform contracted duties without justification.
6. Gross inefficiency that the individual has failed to correct after written notice.
7. Continuing physical or mental disability that renders the individual unfit or unable to perform the individual's duties.