Ticket scalpers, familiar outside rock concerts and sports events, have invaded unlikely turf: a dance for middle-school students. The dance scheduled for Friday night was canceled after the Vernon Junior Women's Club discovered that the $3 tickets were being sold for as much as $10 apiece by student scalpers. The league sponsors dances with the Parks and Recreation Department. A limited number of tickets are sold because the dances are held in a gym that can accommodate only 250 people. There are about 1,100 middle-school students in town. That situation led to students' duplicating tickets and now selling them at inflated prices, club President Peggy Jackle said. She said she hopes the cancellation of the dance will serve as notice to the scalpers that the future of the dances depends on their conduct.