I would definitely say "as".
I find it difficult to analyse/explain exactly why, but:
I think "as" implies that mathematics is (or can be a creative activity), and hence
mathematics is within the group of creative activities.
On the other hand, mathematics is not "like" a creative activity: it either is one or it isn't one. With "like" you are comparing mathematics with another (parallel) activity, not with a group which contains it.
With "like", you are effectively saying "Teachers ... approach mathematics like [they approach] creative activities."
With "as", you are effectively saying "... approach mathematics as [if it were] a creative activity.
I think I would use "like" where there are two parallel, but separate, activities being compared, but "as" where one activity is being compared with a group within which it fits.