If ours are anything to go by, local Education Committee minutes will record teachers' appointments, resignations/retirements, time off sick or at training courses etc But they're very unlikely to be online and you'd have to spend some time at the local archives poring over the books which may not even have indexes. This assumes she worked in a local authority "elementary" school. In the above period what was called "higher education" (County/Grammar Schools, Technical Institutes, etc.) was managed by Kent Education Committee whose records should be at Maidstone archives. Having identified the school and if it still exists, it may have old records or school magazines, though schools have been known to destroy everything on becoming academies.
All bets are off if it was a private school. This is a distinct possibility, as you say she was widowed and women normally had to resign from local government service on marriage. This requirement was beginning to relax by the late 1930s, especially during WW2, and didn't seem to apply to supply teachers. But it does mean that when going through LEA records you should look for a resignation on her marriage date as well as on retirement.
