I'd recommend looking for a prom-esque dress (prom sites work) but short! Not long. See if the prom sites have a "Cocktail dress" division, or search for "cocktail dresses" on google. Save long dresses for prom. I'd also recommend steering away from black/dark-colored dresses. White/pastels/light-colors are perfect for a springtime dance.
Here's what I mean:
Valentine Strapless Cocktail Dress - White
This dress is also super cute in black but as I said before, I'd recommend going with a lighter color.
What Tomorrow Brings Cocktail - White
Again, this is also cute in black, but I'd recommend against it.
Merry Cocktail Dress - White
Merry Cocktail Dress - Red
Rosalind Cocktail Dress - White
Raining in Paradise Cocktail - Green/Jade
Short Pleated Skirt Sherri Hill Dress (available in other colors)
Strapless Short Red Party Dress by LA Glo (also in red)
Strapless Beaded Party Dress by Mori Lee 9210 (also in pink)
Short High Neck Dress
Good luck all you Freshmen and Sophomores! I hope this helps:)