You can't sit down in Palesa Jackson's party dress. Its petticoat is stuffed with about 50 balloons. Jackson, who's in the School of the Art Institute's fashion program, readily admits that it's not practical. "It would be hard to hang out in," she says. "But it's a stage piece. It's an idea." Jackson designed the satin frock--which has "Happy Birthday" spelled out on its panels in eight languages--for an international competition in Paris in which she was one of ten fashion students chosen to represent the United States. Professional models will be decked out in the party dress, five other Jackson creations, and more than 50 other students' designs at shows benefiting the school's fashion department at 6 PM Thursday, May 2, and at 2 and 7 PM Friday, May 3. Tickets range from $15 to $25. Call 899-5158 for more.