I think it's almost certainly just because the teacher has heard "very much <adjective>" a lot, and nearly never heard "<adjective> very much". People tend to copy what they hear/read. The media has (and all of us have) a habit of recycling the same groups of words. It's entertaining to watch out for cliches like "inextricably linked". No two things are ever just "linked"; they're always "inextricably linked". :)

On the other hand, you do hear the pattern "I wasn't surprised very much" or "I wasn't upset much'. So there seems little logic to where you're meant to place the modifiers.

Personally, if there were logic to it, I'd expect the logic to place the modifier before the adjective so that you knew immediately *how* <adjective> we're talking. If the modifiers come later then at first you see just <adjective>, and you might think "ok, a bit <adjective>". And then when you read the modifier, you have to reappraise your opinion to something like "very much <adjective>". :)