the next time you guys are shopping pick up a cute one and show her how gorgeous you think it is ask her if you can try it on and she will see that you want to wear it then show her how it looks on you she will see you in it and see your self esteem an how excited you are
Say something like most girls wear skirts in school during a convo and maybe she'll ask you if you want one.
Unless you have a specific one in mind, head to your local thrift shop. You can find one there for less money which should ease your mom's fears.
Do you have other skirts you can wear at home to show her you would wear skirts? Also could you look for a cheap one so she doesn't feel like she's wasting too much money if you didn't wear it?
juat ask