Hey man I'm a loose follower of MM and I can vouch that it does work just as well in HS for girls that you don't know.
The problem is, most people already know the girls or have a rep with these chicks so it's hard to change that view into an intimate one.
What I've found works is, at the beginning of your attraction, throw in a lot of DHV spiked stories (not all in one day) and neg A LOT. HS girls are all ADD in the relationship side of things and if it gets boring she'll move on. I generally like to work the A1-A3 like norm, go into comfort, then switch back and forth between the attraction and comfort stages to keep things fun.
To answer your question, you should definitely isolate by taking her to a movie, or to watch a movie at your place (if you think she's attracted enough) and start to escalate. Waiting too long for a first kiss in HS is deadly.