Does anyone know why 'nawr' is marked wrong as a translation for 'now' in this sentence?
No reason! nawr, rŵan have both now been added to the list of acceptable answers, although it may take a little while for them to go live.
Is there a way to do this one with "sy"?
You could ask:
expecting to hear one or more names or 'nobody' in response. But if you want the identity of a particular head-teacher or head-teachers then you do need to use Pwy ydy/yw...?.
Hmm. Do you lose the definiteness this way? "Pwy sy'n brifathro" vs "Pwy sy'n y prifathro"?
*Pwy sy'n y...? is ungrammatical.
[nods] And indeed it dinged me for it. :P So there's no way to do this with "sy" without losing that "prifathro" is definite.