Just because she goes to the same school as your friend doesn't mean that she likes him more. From what you're saying, she really does like you - especially as she's the one asking you to get dinner after work. When you go to get dinner after work is it normally just the two of you or part of a group? If its as a group, how about suggesting just the two of you go to get food elsewhere.

The trick is to not go too fast and not too slow. Too fast would be acting all "loved up"; calling her all the time, telling her how much you care about her etc and that will scare her off. Too slow is not making any kind of move, not trying to escalate things from a blooming friendship into a blooming relationship. If you wait too long your chance is blown and you are just seen as a guy friend. You have to find a balance between the two. The best thing to do is to be slighty ambiguous about your interest in her. Ask her out on semi-dates (so one-on-one meetings, but nothing as formal as going to a restaurant etc) and show interest, try to hold her hand as you are walking about, be flirty with her, but don't give it all the "you're the most amazing person ever" type thing.