On and in have many definitions. Some of them from Webster's Third New International Dictionary and the American Heritage Dictionary (AHD) are

on :
with regard to, with reference to, with relation to ---(W)
concerning, about ---(AHD)

in :
with reference to, as concerns ---(W)
within the category or class of, with reference to, as regards ---(AHD)

So either in or on would work.

In the contexts of these sentences, additional and extra are virtually identical synonyms. Pick one.

It may just be me, but I don't think on or in (fractions or geography) is the best wording of what is meant. I might avoid choosing one by rewording the sentences:

She needs to practice fractions more.
She will be given some additional geography worksheets.