How to help

Talk to your friends that have mood swings or are ignoring you because talking to them might not only show that they care but to help understand the troubles your friend is having.
When your friend lashes out at you, don’t take it personally. Your friend could just be going through tough times. Don't get mad or snap back.
If your friend is lashing out at you, give them some time to themselves and let them gather their thoughts about the situation. If it continues for more then a week, you should probably talk to someone about your friend.
If your friend is not looking okay, try talking because it at least lets them know that somebody cares.
If you are worried about your friend, talk to a trusted adult. It can be a parent, a teacher, a coach, or any adult you trust.

When a teen is getting older, they are not only going through changes but learning what they are supposed to be doing and why. If you are alerted of your teen having stress, then you should try to talk to them, but not in a sympathetic or angry way.
You may have also experienced some of what your teen is going through, so try to say what you would want somebody to say to you.
Talk to the guidance counselor at the teen's school or a doctor if you are unsure where to start. It isn't good to be invasive or protective, but the teen should know there are people in and out of school who are there for them if they choose.
Make sure to watch them and monitor if their health, moods, or behavior changes.

Talk to your friend or teen to see what is really bothering them. It is probably not really you, but an effect of the stress causing them to seem mad at you.
If you really are the cause, try to not judge them or make fun of them. Try to still keep in contact and talk to them regularly.
Don't try to start being really nice all of a sudden, because they will know it isn't real.
If there is something you had been doing different recently, think about if it affected your friend or teen. If you think it did, ask them if there is something they would prefer you did differently. You don't have to base your life around it, but maybe stop talking about how good you did at a competition around that person.