Found this on an (expired) job listing for a Swahili teacher in Dar es Salaam:
When serving as Teacher On Duty (TOD), additional responsibilities include:
• Inspect the premises, kitchen, library, dormitories, classes, and offices for cleanliness
• Ensure the availability of clean drinking water
• Supervise the production of food for punctuality, quantity, and quality
• Address illness or other health issues
• Grant permission for students to leave HSS
• Document all of the above in the “TOD” log book
The job listing that @juryrigging posted outlines it pretty well. From my experience teaching in rural Tanzania, the position is basically a product of the underfunding of public schools. The TOD takes on a lot of the logistical duties that in most better-funded schools might be taken care of by secretaries, admin assistants, or other school staff. Usually it's not a permanent position, but rather a position that rotates on a weekly basis from teacher to teacher (or at my school it was a pair of teachers each week).
I see that you are learning Turkish. A teacher on duty is like what they call nöbetçi öğretmen in Turkish.
The word "duty" in East African English is from a British military term meaning "while on the job". It also exists in South Asian English. To be on duty in the army and police meant to be at work and therefore sober, upholding, helpful and lawful. Having said that the correct English sentence should really be "Emilian is the teacher on duty".