So how could I impress her AND her friends?

The age difference isn't really that big of a deal. Just be yourself. If you like her but your shy start slow.. start giving her alot of hugs to show you care and then start giving her hugs with a kiss on the cheek. it's the little things. oh and wear what ever your comfortable in because when you feel good about what you have on you feel good and your confidence will exude through you