I am not a teacher.
I think this is too short. You should let the person reading this application know why you would be good at the job. Tell them something about not only your work experience, but possibly other interests which would make you a good candidate for the job.
To tell them that you retired/left the other job because of "some circumstances" makes me think that there was something unpleasant. You do not need to tell why you left a position unless they ask for it.
You need to emphasize your strong points.
Expand this letter to include the above, and then post again for us to help you fine tune it!
- For Teachers