Hey there!
Graduation speeches mark an occasion of celebration in which notable people are asked to provide context, meaning, wisdom, and advice that will help and assist graduates as they cross threshold between school and the world of life.
Therefore, you should avoid A. a strictly formal, argumentative tone: it is not a real formal and academic text, for that reason you should
D. Unpleasant remark about a teacher: this is not an occasion for making fun at anybody or reveal personal secrets, and you have to keep respecting your teachers.
Hope this helps.