Question #1 how long is a normal period supposed to last?

Question #2 I've been really stressed with school, I have 2 F's in the classes I need to pass, and I haven't had a period in a couple months or so, can this be a reason why?

Question #3 Should I start with tampons? I'm 13 and I was thinking about going out for a sport and thought it would be hard to run and jump in a pad.

So, If anyone could answer my questions, that would be helpful :-)