Which would most a student improve school performance, use time effectively, and decrease stress? increasing self-confidence having good study habits gaining lots of friends being smart

I would say that having good study habits would be key to using time effectively and decreasing stress as with such habits the student knows that through regular hard work they can grasp the concepts and solve the problems. If they are very unsure about something they can ask fellow students or get a tutor for specific questions.

Having good study habits would improve your school performance.