No one believed 11-year-old Bettina Anderson when she told her teachers at Palm Beach Day School she was having lunch that day with Joe DiMaggio.”We were like, `Yeah, right . . . ,”’ recalled Neal Piltch, head of the private school’s upper campus. A few hours later, as 300 students were about to change afternoon classes, a station wagon pulled up to the Seaview Avenue entrance. A door opened. A white-haired man who must have looked like a chauffeur got out and opened another door. Out came little Bettina, who shook the man’s hand and headed back to her fifth-grade classes that day last June.