Yearbooks are published at the end of the school year. So at Christmas 1977 the yearbook had to be for the previous school year. If she was 15 going on 16 she wouldn't be in that yearbook?
I think there is a year (+/-)between the alleged encounter and the yearbook signature. If true, there has to be some kind of corroboration regarding their connection.
At 16, she would not have been a senior unless she skipped a grade or two. So why would she have him sign her yearbook if he had assaulted her a year or two previously.