I suggest we take part in a workshop. Why don't

16. I suggest we take part in a workshop.
Why don't
17. Let's join the English Speaking Club with me.
=>How about
18. It is not necessary for students to wear uniforms on weekends. (not have to)
=> Students
19. It is not a good idea for you to study at night. (shouldn't)
=> You
20. Students are not allowed to wear short jeans to school. (mustn't)
> Students