From my experience, people tend to leave them alone and instead they focus on the competing extroverts for attention.
Well my school was a middle/upper class school with a lot of introverts. It was also the best school in my county with around 5 students gaining a place at Oxbridge each year.
I guess if you're in a school in the inner city, you might be bullied but I'm glad I've never experienced this as an introvert myself but I know many who have.
Introverts are likely to be bullied if they're noticeably different in their attitude and appearance to others but then again, so is anyone introvert or not. I think it's more about social expectation. If you're more attractive than the extroverted 'bully' you might quietly intimidate them by sitting nearby.
But I think it's more to do with personality than simply being introverted and extroverted.
So, I think introverts don't get bullied more easily than anyone else. I do, however, think that being an introvert in conjunction with an undesirable personality is what would get you bullied by others who might disagree with your personality but I think the introvert part isn't as important and is a byproduct of a personality type.