Does anyone have advice on my sleepover problem?

I'm almost 15 and I've been hanging out w/ the new grl at my school and now we're friends. So she inivted me to her sleepover and thats bad because i have a big chest (above a C-cup) and sometimes my tend to look like they are falling out of my pj top when I lay down or bend down. they're not actually falling out, but it looks like it. I didn't really care before because i didn't think any one would notice or care, but at other slumber parties, girls stare at them when they think I'm not paying attention and the girls make jokes and laugh about my s behind my back. I really want to be friends with the grl (she's really nice), i really want to go to the party, but i don't want my to stand in the way, and i don't want to face another night of humiliation for my . Any advice??? any1 have something like this happen to them b4??? please share.