I know what to do, but I can't do it?

You are just shy and I suspect you really like her but are too shy to ask her out. Start being nice to her by waving or saying hello. Then sometime you have a minute to talk ask her to join you for a cup of coffee, a soda or a sandwich. We all have to eat ya know. If things go well before you separate tell her we should do this again sometime and make arrangements to do so. If you wait to make arrangements until later you will be too shy to do so. Hell I should know cause I was so shy I didn't have a date until I was out of school for 2 years!