Meaning of "feet teaching"

[Jupiter]

Hi teachers,

Please look at this sentence.

I feel sorry for poor Martin, on his feet teaching all day and he has to starts work at 7:00.

What does "feet teaching" mean?

Thanks in advance

Jupiter

[Mister Micawber]

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Nothing. He is on his feet (i.e. standing) and teaching all day.
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Native English teacher at Mister Micawber's