From my memory of the extensive revision I did for me VCAA exam - I do not recall "teach" as being an actual design purpose - if it were to have been ever used, then it was never in competition with "inform" and therfore we could have safely assumed enough overlap between the two terms to answer whatever pertaining question adequately.
That being said - in the unlikely event that you do see both these terms in competition over a question (maybe in a SAC, unlikely in an exam) - then to inform is a very simple purpose with the intention of telling someone something they did not know - I would disagree with the idea that pieces with the purpose of informing require pre-existing knowledge, I would think the opposite of this would be true.
Teaching would be different as a purpose - it would be more patronising and it would be more procedural,
Examples
A poster about a sale - inform
A poster about what to do in an asthma attack - teach
In all honesty though - I would strongly believe that the overlap between the two terms is so great, that in any decent SAC and certainly in VCAA exams I did at my time, these two would mean the same and "inform" would just be the more recognized way of saying it.