Can I say "very beautiful" or is it incorrect? My English teacher (actually she's not a native English speaker) says it's a mistake. I've seen examples of this usage on the internet, though.
'Very beautiful' is perfectly good English. The only explanation I can think of for your teacher's comment is that she may be thinking of pairs of words like 'angry' and 'furious', where we can say 'very angry' but not 'very furious', because 'furious' has an 'extreme' meaning to it. 'Beautiful' is not (grammatically) an 'extreme adjective'.
'Totally beautiful' and 'absolutely beautiful' are much rarer phrases, and sound more informal.
If you used very beautiful and absolutely beautiful with a native speaker of American English, there would be nothing unusual or strange in what you've said. These are normal register.
But if you said "totally beautiful" you would have to add a "dude" somewhere :-) That's how Garth and Wayne talk in the movie Wayne's World.
Totally awesome, dude.