That would have been the 1978 yearbook.
The only interpretation that makes sense is, it was the 1977 yearbook, which would have come out shortly before the graduation of the Class of 1977 in June, 1977.
For some reason, this girl was still getting her yearbook signed by newly minted Deputy DA's in December of 1977.
Normal behavior is, the yearbook comes out, you get your friends and favorite teachers to sign it, and then you forget about it and scatter to the four winds. You are not carrying it to restaurants in December to get it signed by 30+-year-old DA boyfriends!
Unless you are a star-struck girl with a crush on an older authority figure...