Mark Sanchez dropped some bad news on Southern California. Defending national champion Florida said goodbye to one star while welcoming back another.

The deadline for underclassmen to make themselves eligible for the NFL draft was Thursday. Sanchez made the biggest news, deciding to go pro after starting only 16 games for USC.

Versatile receiver Percy Harvin gave up his senior season at Florida, no surprise considering he stands a good chance to be picked early in the first round.

All-American linebacker Brandon Spikes is coming back for his senior season to help Tim Tebow and the Gators try to repeat in 2009.

Florida will be a heavy favorite to win a third national title in four seasons, and a virtual lock to be preseason No. 1. USC will be expected to contend for a title again, but the Trojans’ outlook is far more uncertain without Sanchez.

Also announcing they were leaving school were LSU receiver Brandon LaFell and Ohio State defensive back Donald Washington.

Coming back for their senior seasons were Clemson running back C.J. Spiller, Mississippi defensive end Greg Hardy and Georgia safety Reshad Jones.

Sanchez’s decision to leave USC came as something of a surprise and was against the advice of Trojans coach Pete Carroll, who said the fourth-year junior would have benefited from playing more games.

‘‘Mark is going against the grain on this decision and he knows that. He knows that coming out early is a tremendous challenge for a quarterback and the statistics don’t back up that it’s easy to be successful in the way that he’s going about it,’’ Carroll said.

‘‘We hope this works out beautifully for Mark and his family.’’

Heisman Trophy winning quarterbacks Carson Palmer and Matt Leinart both passed on a chance to leave USC early under Carroll.

‘‘He’s a competitive guy, a guy that’s willing to take on this challenge in a way that he’s going to make it happen,’’ the coach said about Sanchez. ‘‘Mark clearly has the potential to be as good as any of those guys we’ve had.

‘‘All it is is an experience issue.’’

Sanchez capped his career by going 28-for-35 for 413 yards and four touchdown passes in a 38-24 victory against Penn State in the Rose Bowl.

He passed for 3,207 yards, 34 touchdowns and 10 interceptions in his only year as USC’s No. 1 quarterback.

Sanchez said he had carefully weighed his options.

‘‘It was with a heavy heart that I say goodbye to this university. But I can’t tell you how excited I am for this dream to come true,’’ he said.

Sanchez ranks with Georgia’s Matthew Stafford as the top prospects in a relatively thin class of quarterbacks for the upcoming draft.

With Sanchez gone, Mitch Mustain, who transferred from Arkansas after going 8-0 as a freshman starter in 2006, is expected to compete with Aaron Corp, Garrett Green and incoming freshman Matt Barkley for the Trojans’ starting QB job next season.

Harvin probably won’t have to wait long to be selected come draft day, either. He gained 1,304 yards rushing and receiving and scored 17 touchdowns this season.

‘‘Percy is the most dynamic player I’ve ever coached and can change the game on any given play,’’ Gators coach Urban Meyer said. ‘‘We wish him the best of luck and we look forward to having him around our program.’’

Harvin’s only issue in college has been staying healthy. He missed time during his three seasons with hamstring and ankle injuries, a stress fracture and migraines.

‘‘When it is all said and done, I wanted to win. I wanted to be a part of championship teams. We did that at Florida and I had a great group of coaches, teammates and medical and strength staff that helped make that happen,’’ he said.

Spikes’ return means Florida’s defense, which held Oklahoma’s record-breaking offense in check during the Gators’ 24-14 victory in the BCS title game, should be back intact next season.

Spikes had 93 tackles, eight sacks and four interceptions this season.

‘‘It was a tough decision for me and my family,’’ he said. ‘‘I just felt like it was the right choice to come back.’’

Spiller, who had 1,770 all-purpose yards and 11 touchdowns this season, received a first-round grade from the draft advisory board. His mother also wanted him to turn pro.

‘‘I know a lot of people are wondering, ’This guy crazy?’’’ Spiller said. ‘‘I know everything will work out.’’