It's HS we're talking about remember, hell if the girl was into you, asking her out in some clownish way like over IM could even work. (Don't ever do that though!)
What's with the bull**** about losing interest, you saying if you were crazy about a girl and for some strange reason she decided to ask you to be her b/f you would say no and lose all interest in her?
Infact when I once asked a girl out, even though she said no, she started showing a lot more interest in me, you know why? because she was probably thinking about the fact I asked her out for a long time that night!
Trust me, there is nothing better than a girl thinking about you when you are not around, because that is when attraction goes crazy,you drive her insane!
Know how I figure this out? not from any seduction guru or seminar or ebook. I simply thought about what happens when I fall for a girl. She might do something or we might have a nice convo at school or wherever and then later that night I can't stop thinking about her and... BINGO I'm attracted. That is the core of attraction if you ask me.
I might be wrong but whatever, I believe it.
p.s Make sure when you do it, you drop it as more of a suggestion and don't take it too seriously or start begging or something lame (yes ages ago in my afc days I may have actually done that to some extent