Thing to remember is that, Welsh is a very old language, before the revolutionary Women's Right Movement. So we assume it's male, until specified.
Although, as in most/all other countries, teaching is predominantly (seen as) a women's profession in Wales - so assuming the female gender would seem more logical in this case...
If the gender is unspeecified or known to be male, use athro, if known to be female, athrawes.
In this sentence either athro or athrawes can be used.
Oh yes, I'm familiar with the "if unspecified, use the masculine noun" concept - I'm simply pointing out that for traditionally female-dominated professions, the reverse would actually make more sense :)