Most Helpful Girl
I don't know why you would want to do this, but ok. Girls will easily do it for you (it's pretty fun to make guys up), but if you aren't close to them and you're afraid they'll find you weird, here are some excuses:
1-) The most simple excuse: "lol, I just wondered what I'd look like with make up"
2-) "There's this costume party and me and my friends are going to show up as girls. It'll be hilarious"
3-) "I lost this bet and now I'll have to show up as a girl at this party :/"
4-) "It's a prank. I'll tell my dad I'm a girl trapped in a guy's body and I want a sex change surgery. He's super conservative so it'll be hilarious"
Now, these are excuses that are less general (you'll have to use them in an specific time of the year):
5-) "My friends and I will go trick-or-treating as girls. It'll be hilarious, my neighbor is super conservative so he'll freak out."
6-) "It's the last week of classes, so tomorrow everybody will show up at school as their opposite sex."
Most Helpful Guy
WOW , seriously just WOW