...or should I say "yesterday mathematics teacher"?
Usage in a full sentence:
'I'm waiting for yesterday's Mathematics' teacher to come online'
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American here, so your usage might vary by country. I think it will be dependent on your intention.
Yesterday, the mathematics teacher taught algebra. Yesterday's mathematics teacher taught algebra.
I think both are fine but mean different things. The first one means that the normal math teacher taught algebra yesterday. The second infers that there was a different teacher (substitute teacher possibly) yesterday that taught algebra.
In your example, I think this would be what you want to do:
I'm waiting for yesterday's mathematics teacher to come online.