Check out what guys have to say on the subject:
"What's appropriate can vary in different situations. It would normally be immodest to turn up to, say, school or church in a leotard, but there can be appropriate contexts such as artistic or sporting performances." - age 21"These sorts of clothes are fine in their context. Being distracted by them is a problem on the guy's end, not the girl's." - age 19"I'd say, if a girl can avoid wearing such outfits, it would be nice if they would." - age 26"I think that doing anything with the intention of showing off the body can be considered immodest." - age 19[responses from therebelution]
I think the biggest point I could make here is that when your heart is in the right place, everything else will fall into place as well. Make sure the dances you perform and the costumes you wear are done to show off your ability and not your body.
To tidy up your costume, politely ask your dance instructor if you could wear a tank top under your crop top. You can find them almost anywhere and they're pretty inexpensive. (I got mine at Target in several different colors.) If you're worried about showing too much leg, perhaps you could wear tights or leggings under your shorts, or opt for a longer pair of shorts if possible. Be sure to explain to your dance instructor why you wish to tweak your costume. If you're worried she'll say no, pray that God will work things out before you confront her. Have fun dancing!