"Ten uczeń ma trudną osobowość."

Translation: This pupil has a difficult personality.

[diftcsx]

Are many nouns ending in -ść feminine, or just some of them, or most of them?

[Jellei]

My guess it's that it's most, but then you surely have "gość" or "liść".

[Jack.Elliot]

this pupil has a difficult character

http://www.thesaurus.com/browse/personality

[Jellei]

OK, added.

[kileli]

Mm...I guess it's a word-to-word translation, but as a native English speaker, that doesn't sound very natural to me.

[anetagh]

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.