The 86 seniors of Cohasset High School graduated in style last weekend at the South Shore Music Circus. Late last month, the students at the senior prom showed even more style when they walked a 50-foot red carpet and posed for pictures in a community tradition. Making the tradition a little more special this year: The red carpet was donated by actress Kate Bosworth, a 2001 Cohasset High alum who played Lois Lane in the 2006 movie “Superman Returns.” “We wanted a longer red carpet so kids could be on it longer and more spectators could see them,” said parent Elizabeth Kissick, who is a friend of the actress’s mother. “I thought red carpet? Hollywood. I’ll ask Kate. So I asked, and she generously said yes.” As another part of Cohasset’s senior traditions, the prom-goers rode trolley buses into Boston for the dance, and then back to a parent-supervised “After Prom Safe Party” at the River Club in Scituate.