I hate it when people feel the need to stand up and throw something away in the middle of class.
There are times when this is acceptable. If it's a long class where there is a lot of moving around and/or individual work, you can definitely get up to throw something away, especially because you're often allowed to eat in those types of classes.
Also, if, during a mostly lecture-based class, the professor gives you a few minutes to read something or write something in response to what you've been talking about, it won't be incredibly disruptive for you to stand up and throw something away.
But honestly, I still think it's rude in the second example.
What's completely unacceptable is when people stand up in the middle of a lecture-based class in a small classroom and walk right past the professor to throw something away.
What is the point? Why did you feel the need to get rid of that tiny piece of paper? Just set it aside until class is over and throw it away then! Problem solved, disruption avoided!
Of course, the girl who made me want to write this post in the first place is also possibly the least self-aware person I've ever encountered.
An example: She takes off her tennis shoes in the middle of class and puts them in the middle of the aisle, like where someone would have to step over them to get past, which is gross and rude.
So of course, she probably doesn't even realize that she's being disruptive. She's off in la-la land, skipping past the professor to get to the garbage can and being utterly confused when he gives her a weird look.