My youngest wears one green outfit every other Shabbat – Friday night and Shabbat day. She even goes to shule in that same green outfit, and has, on occasion, gone out for meals dressed that way. (Why green? She closes Shabbat every other week, so she’s on base.)
If the school has a one outfit rule for a shabbaton, I’m sure the young ladies can manage for one week. It’s not the end of the world; after all, it’s not like you’re going on a date.