Miniskirt doesn't fit??

Hi i'm 21 and my bf has got some pleated elastic waist knee-length and miniskirts for my birthday. The problem is i'm 5 f 6 and whenever I sit wearing these pleated cotton skirts without adjusting the backside the backside of my skirts ride up and leaves a part of my s exposed to the chair. I told my bf i felt like a slut in it because it leaves me back exposed and he said don't worry babe you just have to get used to it. I haven't worn pleated skirts before so I don't know. It's worse with the miniskirts. Could you please clarify if you're a girl and you know?