I wouldn't ask a girl out in front of people -- that'd make it pretty uncomfortable.
I also wouldn't take her hand and whisper something to her -- that is essentially asking her out in front of people, only using body language to do so.
Use a pretext. Whatever pretext you can think of to get you two out of the circle of people. If she's into you, you won't even need a good pretext.
Also, the fact that you say there are only three days left in school and you want to ask her out -- that's a big red flag to me, man.
That tells me you've been pining for this girl for some time -- and now that school is almost out, you feel the risk is less to ask her out, because if she says no, then you won't have to see her around, right?
That's the worst mistake to make.
But go ahead and make it. (I did the same thing once, on the last day of class, in college, I walked out with this girl, and then as we parted I asked her if she wanted to see a movie. Well she had this guy she was kind of seeing, she said. A complete lie -- she wasn't into me at all because she'd known I was into her from day one and didn't make a move and instead waited until the last day -- can imagine how much of a turn-off this is to a girl?)
Because I don't think you learn unless you make the mistake YOURSELF, go ahead and make the mistake.
--BGC