Thanks SoftSign.

That makes it very clear: преподаватель just doesn't have the separate accusative. It has to be -Я even if the person referred to is female.

However what happens when the noun is feminine and does have the accusative ending? For example:

личность

Я наблюдаю личность / личности depending on the gender of the 'individual'? Or - always личность because (like the other way round) it is the gender of the noun which counts? I suspect личность in any case?

Thanks again.