Are many nouns ending in -ść feminine, or just some of them, or most of them?
My guess it's that it's most, but then you surely have "gość" or "liść".
this pupil has a difficult character
Mm...I guess it's a word-to-word translation, but as a native English speaker, that doesn't sound very natural to me.
I'd rather stick to 'personality'.
There is a word 'charakter' in Polish and I would use this one if I meant "character", I guess...
The two are synonymous only to a degree.