No, you do not need a passport to go to the USVI (U.S. Virgin Islands) if you are a US citizen. The USVI (U.S. Virgin Islands) is a US territory, and US citizens do not need a passport to go to the USVI (U.S. Virgin Islands, including St. Thomas, St. John and St. Croix) or any other US territory -- traveling to a US territory from the mainland United States is the same as driving from Texas to Oklahoma, or taking a plane from New York to Seattle. You're still within the United States' legal jurisdiction.
You do need identification, and you may wish to have a birth certificate to prove citizenship, when you travel to the USVI. The US Customs and Border Patrol has this
to say about documentation needed for travel to and from the USVI:
"Although U.S. citizens are not required to present a passport upon departure from the U.S. territories, travelers are encouraged to travel with a passport or other proof of citizenship, as they will be asked questions about citizenship and any goods they will be bringing to the U.S. mainland upon their departure from U.S. territories."
See more about US territories, passports, spring break, and all that good jazz:
Everything you need to know about passports: where, when, how, and what:
And everything you need to know about spring break -- most especially where, when, how and what (and how much!):
US Territories, Drinking Ages
US commonwealths/territories are American Samoa, Baker Island, Howland Island, Guam, Jarvis Island, Johnston Atoll, Kingman Reef, Midway Islands, Navassa Island, Northern Mariana Islands, Palau, Palmyra Atoll, Puerto Rico
, US Virgin Islands (St. Croix, St. John and St. Thomas), and Wake Island.