If you are unable to pick up your yearbook yourself, here is a sample template you can use to write an authorization letter for a yearbook.
I, [Name], am a student at your school studying in the class [class details]. This letter to bring to your attention that due to unavoidable circumstances, I am unable to claim my yearbook myself. This is why I hereby authorize my [relation], [Person’s Name] to collect the school yearbook on my behalf.
I have asked [him/her] to bring identity proof that you can use to verify and authenticate [his/her] identity. So, I request you to cooperate regarding the same and allow [him/her] to claim my yearbook for me. For any further concern or clarification kindly contact me at [Phone number].