Girls respond to confidence, and 17 year old girls love -- and I mean they eat this stuff up -- to be treated as though they're older.

Feel free to use the following:
"I'd like to take you out to dinner Friday after work."

Say it with total confidence and she'll be yours. Pick a restaurant with tablecloths, if you can afford it.

If it does turn out that she has a boyfriend, you can just say "thats too bad. Another time, then." There's no reason for either of you to be embarrassed that you want to take her out on a date.