Based on the information provided, Leonard's symptoms—withdrawal, agitation when questioned about his behavior, and nightmares specifically about school hallways—suggest that he may be experiencing trauma related to bullying. This could be indicative of post-traumatic stress disorder (PTSD).
PTSD can develop after exposure to a traumatic event, which in this case would be the bullying. Symptoms of PTSD can include re-experiencing the trauma through nightmares, avoidance of reminders of the trauma (which could be likened to his withdrawal from social situations), and heightened emotional responses (such as agitation).
The other options—obsessive-compulsive disorder, generalized anxiety disorder, and panic disorder—do not fit the symptoms as closely as PTSD does in this context.
Therefore, Leonard may be diagnosed with post-traumatic stress disorder (PTSD).