Act like you two have no past.
If the trip down memory lane turns out to be paved with awkward silences, switch it up. Shift into a more standard approach. Because picking up a girl at your hometown bar need not be different from picking up a girl at any other bar.
Act as if the two of you don't have a past. Get to know her now, in this moment, as this person—not whoever she was in 11th grade. Chances are, she doesn't have the kindest feelings about that person. Create chemistry based on where you two are now.