How rare is it for a 12yo to get oral cancer?

Very rare: Epstein barr virus causes a type of oral cancer that may occur in a young person. Usual oral cancers occur in older persons with history of tobacco and and alcohol use.

Squamous cancer: Of the head and neck is unheard of in a pre-teen, and only teens that smoke or dip are at risk. Pediatric tumors (rhabdomyosarcoma) can afffect the head and neck, but the oral cavity is still unusual. Ewings sarcoma of skull bones, and esthesioneuroblastoma and juvenile angiofibroma occur in nasal cavity.

Not common: The type of oral cancer that adults most commonly get, squamous cell cancer would be very rare in a child. There are many minor salivary glands in the mouth, and these can become cancers in children and are called salivary tumors such as mucoepidermoid carcinoma, etc. These and other cancers are possible even in children. I have treated many of them.