Dressing up for others doesn't equate to dressing up for guys. Otherwise, why would girls at all-girls school care about their appearance?! Most girls make an effort because a) picking out nice outfits, doing your make-up and styling your hair is fun b) everyone likes having something pretty to look at when they stare in the mirror c) a lot of girls are appreciative of fashion and style, and you'll find many girls who would be much more delighted by a female friend saying "Wow, I love how you've accessorised that dress, it works so well, and where did you buy those shoes?!" than a guy saying "You look fit". Dressing up is a way of expressing oneself, so it's fundamentally both about doing it for yourself, and for other people, as you can't really be expressing yourself if you have no one to express to.