Some people were talking about me behind my back (literally, at the back of the classroom- I am at the front), and I think someone said I was "mainstream", what would that actually mean? Might it be considered insulting?

It all depends on the tone when they expressed that.

Mainstream usually means that you are conventional and nothing out of the ordinary. Example: You don't wear outlandish clothes.

There's a great site for terminology.  Urbandictionary.com. Check it out. 

I think used positively it can mean you're trendy. Used negatively, it usually means you're boring

Mainstream definition---the ideas, attitudes, or activities that are regarded as normal or conventional; the dominant trend in opinion, fashion, or the arts.

"companies that are bringing computers to the mainstream of American life"

I'm almost 72---I have no idea what it means when used by those who are still in school.