amren, The following can cause hair loss:

Thyroid Disease....get all thyroid levels checked by an endocrinologist. Most of the time primary doctor will not check all of the levels.
Autoimmune Disease can cause hair loss.....Get primary doctor or Rheumatologist to check for this.
Stress can cause hair loss. Has there been a lot of changes in there life in the past year? Stress can cause hair loss.
COVID can cause hair loss. I lost so much hair from COVID. It has taken it a while to grow back. If your hair is shoulder length, its take 2 years for that hair to grow back my doctor said. He was right.
Hormones can cause hair loss.
Lack of certain vitamins can cause hair loss. Are they eating well? Most teens don't eat well and get the daily requirements. Low iron, B12, B9, Vitamin C, Biotin, Zinc, Omega 3, & Selenium can cause hair loss.
Having surgery can cause hair loss.
Extreme weight loss too fast can cause hair loss.
Blood pressure medication can cause hair loss (I am doubtful they are on this).
A Dermatologist a lot of times can determine what is causing hair loss.
Stressing the hair with wearing pulled back tight or up in a bun can cause hair loss. Overdoing chemicals in the hair with too much coloring, perming, etc., can cause hair loss.
Alopecia (please look this up) is a very common malady at any age. A teenage boy in our church had this and patches of his hair were coming out. He had bald spots. It did grow back. A Dermatologist should be able to help with this also if that is the case.
Use a shampoo with natural ingredients and not harsh chemicals.

Praying as I can only imagine how devastating this is to a teenager. Blessings......