My first reaction is ha! good for the professor!My second reaction, as a writing teacher, is that the professor is showing lack of understanding of some basic principles of rhetoric, including the fact that the rules vary according to genre and audience. Using contractions -- even contractions missing their apostrophes -- is probably just fine on twitter (where I don't spend enough time myself to feel fully qualified to opine on the conventions). One could complain about "gunna" (no savings of characters over gonna), but that may be established text/twitter usage. Of course, the student made an even more basic mistake in analyzing the rhetorical situation -- he didn't identify all the members of his potential audience, and frame his communication accordingly.
I always cringe when I see these examples that, as CC has pointed out, DO show a lack of understanding of the medium. That said, students who forget that professors or teachers are also on social networks are seriously lacking in basic common sense.