I take the issues in this order, in a conversational tone (even saying, “You see how we’re just discussing what happened with calm voices?” so he sees and learns how it can be handled without anger or yelling; this lays the groundwork for future conversations):
1) I’m most concerned about the suicide comment. I tackle that first as a learning experience that bad things happen throughout our lives, but it will be ok. And I’m always there to help and figure things out. I love you whether you get 100% or 8%. Is something else bothering you? Bullying? Other stuff? Life is hard sometimes but we work through it and work to do better. What makes you say something like that? Depending on response go deeper with do you really want to hurt yourself or is that just something you said, because hurting yourself is never an answer, etc.
2) What’s happening in science? Let him explain it. What can we do to improve your science grade? Make a multifaceted plan so it can be attacked in multiple ways: more time spent, study methods, rereading difficult text and explaining what it means with you helping, note taking tips picking out important points to write down (multiple forms - reading and writing add up to better comprehension, looking things over again other times, 30 minutes of fun reading per day to develop that skill, etc. These habits will help in other classes too and little successes should be applauded. Life isn’t easy and it will always be more “fun” to not do these things and play more iPad or Xbox, but that’s life. New habits take repetition.
3) Forgery and teacher involvement. Signing for you brings the honesty and lying conversation into play. This is a theme that will also be reinforced in future conversations like “calm” discussions above. Why did you sign my name? We know the reason but it helps for him to say it. Then, I know you care and want to do better because you knew I’d be disappointed in the score. You need to be honest with me and your teacher because we want the best for you. I can’t help you if I don’t know there’s a problem. Remember the story of the boy who cried wolf? If you lie, then when you are really telling the truth people may not believe you. I expect you always to tell me the truth even if you think I’ll get mad. Do you understand?
He should apologize to you (hopefully without you asking) and the teacher, in person, and tell the teacher how he plans (not hope) to do better by doing x, y and z. This reinforces that he knows what methods are available to improve.
I don’t know your school but I’m pretty surprised that a) the teacher fell for the forgery or didn’t care, and b) didn’t call or email you to discuss the low score and concern for your son’s progress. In your own conversations with her I’d for sure reiterate that you support her and the school and would like to stay in touch on his learning and are open to ideas. What does she see?
Keep the lines of communication open with your son and the teacher. GL