I cannot think of a specific law wherein one convicted of misdemeanor Assault & Battery cannot be employed as a teacher. However, at least, it will make getting a job more difficult. There is a concept in the law called "negligent hiring," wherein an employer can be liable if he or she hires an employee who hurts another. Under this concept, your conviction may scare off some of your potential employers.
You may want to post this question to an employment lawyer, someone in that field may have more information about whether or not a conviction would disqualify you. I would also think about checking with the school board to see if they will consider you or not.
You say such a matter is handled on a case by case basis. You may wish to ask your lawyer for the assault case to draft a brief letter for potential employers, putting the assault case in as favorable a light as possible for you. I also agree with the suggestion that you consult with an employment lawyer; when applying to work at a public educational institution, regulations may exist that give guidance on the matter. .
All the best to you. Jon
None of my comments on AVVO's website constitute legal advice. It is only food for thought for matters to address with a qualified laywer. My website's disclaimer page at http://katzjustice.com/disclaimer.htm applies to all my answers on AVVO.com .