Another thing is that most of the people who are bi through out high school tend to make up their mind which side by college.
I dont have any bi friends anymore cuz they picked one.
So for now you can be (I'm saying this seeing as the general population on here are young folks) but I can bet a good amount of money that you will pick being gay or straight right around your college years.
I dont think calling yourself bi in middle school is good enough, or saying that you have made up your mind.
I think if you are in high school I think you almost have the right mind set and decision making capabilities to make that kind of conclusion for then.
So basically based off of my experience being 20 years old:
middle school: No, I dont think youre mature enough to make the full decision but yes you can experiment and call yourself bi...for now.
high school: Alot of people will turn one or the other, but alot of those bi people will go back and forth. I think you can say your bi and mean it. But its temporary until you make up your mind.
College and after: You'll probably pick a main side but at the same time be attracted to the other of course but by then you will decide what you want to live with(significant other) for the rest of your life. You wont be exactally bi but you will be like...75% one way and 25% the other. Which is mature.