okay so you have wednesday, friday, and monday?

take others advice but just try to be friendly with her wednesday and friday, get a little communication going on, make sure she takes notice to you and then on monday when the class is done ask for the number or email or whatver....

its gonna be easier to ask her for her number if you've already talked to her atleast a little bit....you don't want your first words to be "hey whatever your name is, your hot can i have your number?" Play it cool for two of the three days and make your progress then get the number monday...

and a benefit of that is if you ask and she says no on monday, you'll never have to see her again, so you shouldn't have much to worry about

if your a wussie as the guy said...u may wanan try the friendly thing tomorrow and then ask for her number on friday just incase you chicken out you'll have one more day

i myself would TRY to ask earlier then the last possible minute just because i can tend to be a pussy at times (i can't help it )