It may be intentional; it may not. I've known and observed a few long-haired women who either had no concept of where their hair was going or didn't care about violating others' personal space. (e.g., a transatlantic flight in coach with the stringy straw-like graying hair of the smoker sitting in front of me invading my limited space.)

Or it could be intentional. I've known a few women who knew exactly how much many guys liked their hair and knew exactly where the full length of their hair was at all times.

I'm no expert with women, but consider sitting next to her (not behind her) and striking up a conversation about class or whatever (not her hair). Try to work up to asking her out for coffee or a drink or whatever is done where you are. If she seems interested, great. If you get a real first date, maybe try a single compliment relating to her hair, especially if she wears it differently than her everyday look. Hopefully you'll be lucky. Maybe she will even use the compliment to strike up a conversation about her hair. It sounds like there is some potential; women don't generally have well-maintained butt length hair and wear it down by accident or without having any thoughts about it.

If you get to the point of any physical contact, that's when you can maybe begin to think about "sexy hair stuff." A surprising number of women -- though certainly not all-- seem to enjoy having their hair handled even if they don't see it as particularly sexual. And they may go for it and enjoy it significantly sooner than they would let you go to "second base" or farther. Moving her hair out of the way or learning to brush her hair can be great for a long hair fan and be intimate in a sweet and fairly innocent way.

But if she shows any sign of disinterest, it's best to move on and never sit near her again. If she finds out you were intentionally making contact with her hair and/or enjoying it, she might find it creepy or worse.

It sounds like you are in a college class with her, and a real danger is that the U.S. college sexual environment is dangerously hostile to guys these days; having a relationship with someone from your school puts your whole education at risk. Many university "sexual misconduct" policies include "unwanted leering" and "unwanted brushing against the body," presumably including hair. The woman gets to decide what is unwanted and can decide such after a breakup or even years after the incident. If the guy is still at the school, he can be disciplined and kicked out with no due process. Depressing, but that seems to be the reality right now.