In my experience, DOS positions are usually promoted from within. In any case, advertised posts are extremely rare. To be a DOS of a small to medium sized language school would probably not require much more knowledge of how things are done than you would already have picked up by having been employed as a teacher. However, if you feel you would need to acquire special skills in leadership and/or (especially) teacher training or teacher performance assessment, you may find such a course helpful.
If you were thinking of the DELTA, modules 1 and 2, you may wish to consider if there are any foreseeable career opportunities that would provide an ROI on the relatively high course fees.