As long as you weren't completely going through your mother's things without her permission I think if you wanted to discuss this with your mother you would be welcome to. It might be that the signatures in her yearbook are referring to a few specific instance and that being drunk and stoned did not make up your mother's whole high school experience. You should probably hear the story from her before you make assumptions about her past.

Maybe if you don't want to bring up the yearbook you could just ask your mother more specific questions about what her life was like in high school. Maybe she won't want to tell you the whole truth or everything she did but at least you should hear about it all from her. If you feel like really dealing with this topic too much may really upset you then I think it might be best to just ignore it for awhile and know that maybe when you are even a little bit older you will be able to have more conversations with your mother about what her high school life was really like.