How to make my hair look like a a guy's?

HI! okay, so tomorrow is alter ego day at my school, and my sister doesn't rlly know what her alter ego is, so shes going as a guy. buuuuuuuuut! she has loooong blonde hair, and putting it in a pony tail in the back of her shirt looks....odd. so if your guys have any suggestions on how to make it look half decent, id be thank ful! oh! and obviously we cant cut it so....anything but that is welcome.;)