Most Helpful Girl
If she purposely sits next to you often without it being assigned seating she's making the conscious effort to be closer to you, I would usually never tell someone I like them. I would laugh around them a lot, laugh at their jokes, smile at them a lot. Keep glancing at them, I would definitely talk about them to people so maybe somebody knows and they can give you some insight. Also she might not be shy atall it might be you who makes her shy.
Most Helpful Guy
Wanting you to walk her to class is as much of a global broadcast as you'll get with some these shy ones. She is interested, so it's time for you to make a move.