We don't know.
All we know is that Chiron
assaulted him with the chair. Certainly he was injured but we don't know the severity of the injuries.
Given that Chiron is only sent to juvie it's extremely unlikely that Terrel was killed.
Thus a charge of the Assault variety would seem likely but that is also unknown. We also don't know how long Chiron was in juvie but most age out of the juvenile system at 18, assuming he was tried as a minor.
However, there is a 10 year time jump between the assault and when we see Chiron again so any sentence would have been less than that since Chiron has now relocated to another city, is considerably larger than before and is now established as a drug dealer.