I really like this question as a mom of teens! I know I was under a lot of pressure when I was in adolescents but it seems pretty intense these days.
I try to help my sons with organization. Sounds simple but by staying on top of organization, it cuts down on a lot of stress. Shoes where they are supposed to go, know when homework is due by checking frequently, planning out their week, etc. This does seem to help.
I also have taught my kids that we work hard but have to take time for ourselves when we can. They should be able to do whatever relaxes them for a space of time every week. That down time is good for bringing down stress. Works best when we've planned it into the schedule so that they aren't doing their relaxation things in lieu of need to do things.
I have taught my kids how to do calming self talk as well.
I model patience and calmness.
Teens can decrease stress with the following behaviors and techniques:
Exercise and eat regularly.
Get enough sleep and have a good sleep routine.
Avoid excess caffeine which can increase feelings of anxiety and agitation.
Avoid illegal drugs, alcohol, and tobacco.
Learn relaxation exercises (abdominal breathing and muscle relaxation techniques).
Develop assertiveness training skills. For example, state feelings in polite, firm, and not overly aggressive or passive ways: ("I feel angry when you yell at me.” "Please stop yelling.”)
Rehearse and practice situations which cause stress. One example is taking a speech class if talking in front of a class makes you anxious.
Learn practical coping skills. For example, break a large task into smaller, more attainable tasks.
Decrease negative self-talk: challenge negative thoughts - with alternative, neutral, or positive thoughts. "My life will never get better” can be transformed into "I may feel hopeless now, but my life will probably get better if I work at it and get some help.”
Learn to feel good about doing a competent or "good enough” job rather than demanding perfection from yourself and others.
Take a break from stressful situations. Activities like listening to music, talking to a friend, drawing, writing, or spending time with a pet can reduce stress.
Build a network of friends who help you cope in a positive way.
(There are some apps which teach this. 'Headspace', (I forget the other one))
This will help strengthen certain areas of the brain, which leads to stress reduction (and other health benefits).
A few minutes a day for 2 months should show some results.
There's also mindfulness motion exercises: Qi-Gong, Tai-Chi, Yoga (Therapeutic yoga. Too many types of yogas.)
There may be some meditation classes nearby. Or Buddhist places often have meditation classes. (Doing it in a group can help for introverts and people feeling isolated.)