I really don't want to make it secretive but it may be a must..seeing that I REALLY can't ask in front of a group of people...if I can't find a point in which she is alone I have to forget it, that or lose my lunch :p
Well, it's a sort of traditional thing to be someones Secret Valentines.
You send them a letter and maybe a valentines gift and they wonder who it is. You can come out as their secret valentines later on, or even drop hints that it might be you.
It's a sort of romantic game.
And the secretive part is just tradition, so it doesn't have to appear to be a cover up for insecurity.
And if she still doesn't want to go out with you, it's all a bit of harmless fun.
Just don't make yourself appear like a stalker.
Write her a simple soppy love poem and some soppy paper hearts, pop it in an envelope, and drop it in her locker, or school bag when no one is watching.