The term 'crackpot' is informal and might be disrespectful in formal contexts. Don't call your teacher a crackpot; it's rude and too informal.
Pro Tip 2/3
Negative Connotation
'Crackpot' suggests disapproval and might offend someone if used to describe them. Be careful when calling someone a crackpot; it can be taken as an insult.
Pro Tip 3/3
Cultural Sensitivity
Some cultures might find the term 'crackpot' less offensive than others. In some circles, 'crackpot' can be affectionate teasing.