Ah, to be young and in love—because we all know how those romances turn out.

Production on High School Musical 3 is in full swing, and costars Zac Efron and Vanessa Hudgens so far have made the set a PDA-fest.

During a 15-hour shoot in the auditorium of Salt Lake City's East High on Tuesday, sources say when Efron and Hudgens weren't holding hands and laughing at each other's jokes in between takes, they were sprawled out on the front of the auditorium stage "like lovers on a picnic."

"When they're not needed on set, they're on each other's laps," says a set source, adding that the young lovers also were playfully waltzing while costar Corbin Bleu played the piano.

Tuesday's scenes involved the senior students discussing what the spring musical should be called, and security on the Utah set is tight.

In fact, production staff have ordered extras not to approach the actors at all—not for autographs or even directions to the bathroom.

But when all are filming in the auditorium, extras can sit near the actors in the "audience chairs."

That is, unless they're still looking for the bathroom.