A Most common elbow injury in adolescents is-
Some guides have given, supracondylar humerus fracture as the answer. But they are wrong as the supracondylar humeral fracture is the most common elbow injury in children (not an adolescent). Most common age group for supracondylar fracture is 5-7 years.

"Most common site of injury in the adolescent's elbow is the epiphyseal plate (physis or growth plate)".-- Operative treatment of elbow injuries

During adolescent growth spurts, the physeal plate is weaker than the surrounding bone, therefore it is the most common site of injury.
Dislocation
Physeal injury
Supracondylar fracture
Olecranon fracture