Aphthous Ulcer: This is the most common cause of mouth ulcers in adolescents, characterized by recurrent, painful ulcers on the mucous membranes. The exact cause is unknown, but stress, trauma, and certain foods can trigger them.

Herpes Simplex Virus (HSV): HSV can cause painful mouth ulcers, especially during the initial outbreak. It's highly contagious and can be spread through skin-to-skin contact.
Hand, Foot, and Mouth Disease (HFMD): This viral illness is common in children and adolescents, causing mouth ulcers, fever, and a rash on the hands and feet.
Trauma: Accidental bites, dental work, or other forms of oral trauma can cause mouth ulcers in adolescents.
Nutritional Deficiencies: Deficiencies in vitamins like B12, iron, or folate can lead to mouth ulcers, especially if the adolescent has a poor diet or certain medical conditions.

Oral Cancer: Although rare in adolescents, oral cancer can present with mouth ulcers that don't heal. It's crucial to rule out this diagnosis, especially if the ulcer is persistent or accompanied by other symptoms like weight loss or difficulty swallowing.
Infectious Mononucleosis (Mono): Caused by the Epstein-Barr virus, mono can lead to mouth ulcers, fever, and fatigue. It's essential to consider this diagnosis, especially if the adolescent has been exposed to someone with mono.
HIV/AIDS: Mouth ulcers can be a symptom of HIV/AIDS, particularly in the later stages of the disease. It's crucial to consider this diagnosis, especially if the adolescent is at high risk or has other symptoms like weight loss or recurrent infections.

Behçet's Disease: A rare autoimmune disorder that can cause mouth ulcers, genital ulcers, and inflammation in the eyes.
Pemphigus: A group of rare skin disorders that can cause blistering mouth ulcers and skin lesions.
Erythema Multiforme: A rare skin condition that can cause mouth ulcers, skin lesions, and other symptoms, often triggered by infections or medications.