highschool get her number?

This girl ive posted about before and i have changed seats and im in the front and she is in the back. Anyway, she has been in a break up about a little over a month now and i want to get her number. We dont talk anymore as much except maybe a simple hi and smile wave or just acknowledgement. Because of this, im not sure how i should approach her and ask for her number without sounding desperate. I sometimes see her during the halls (ive pretty much mapped out where she usually is) and today she wasnt here at school. When i see her what is the best "line" to ask her for her number? Surprisingly, i never asked for a girls number except if it was for like a school project but in math class, there is no project so its kinda hard. What would you say?

I'd approach her with something like "hey, would you like to go out with me sometime?". If she says 'yes' then ask for her number so that you can call and arrange the date.