Stress.

In high school, there will be points in the year where stress becomes a chronic state, rather than just an adjective used to describe yourself for one particular day. Some of these instances will be in the crunch time of your sport season, the weeks leading up to finals, and when your friends are all in a spat. As someone who gets overly stressed and has had multiple weeks filled with stress induced stomach pains, one may think I am not qualified to give advice on such topic. I, however believe that I am fully capable and will proceed to write this anyways.
In any instance where you feel stressed, you first need to analyze the situation at hand. You need to determine if the problems you are faced with are really worth loosing any sleep over. In most cases, they usually are not. Getting worried about a big race, meet, match, or game will only keep you from doing your best. Studying all night rather than getting a good night's sleep the day before a test may seem to work, but for finals, there is not enough hours in a night for it to work. If you do not know the material, there is little you can do the day of, or in the wee hours of that morning. Allowing an argument between anyone that does not involve yourself, is a waste of time. That does not mean do not care if others come to you with their problems, listen and give your best advice while always staying neutral. When you get involved, things never turn out the way you wish them too. My best advice, is to stay out of others' petty personal problems. Of course, this does not include situations where right and wrong is being questioned. In those instances you should always firmly stand with the side which you believe is right.
One way to prevent yourself from being stressed is to stay very organized. Make yourself a schedule and keep it up to date. Constantly put things you need to remember on a To-Do list. The times we usually get stressed out in high school are for reasons that could have been avoided. When I get stressed out, it is usually because I have let myself believe that I can be in two places at once, but sadly, they have yet to invent that technology. Do not allow yourself to get behind or let your responsibilities become cluttered in your head. This is the best way to avoid stress in general.
Even the most organized people still get stressed unfortunately, and even if you follow my advice to avoid stress at all costs, it still may happen. It will happen. We are in high school, and stress in inevitable. Do your best to roll with the punches, just start laughing whenever you can, and look at the bright things in life. Never let stress get in the way of you being happy because I have yet to think of something that should be placed over you having a good day.