He attended public schools until he was 14.
I cannot understand this sentence. Please tell me if:
1)this is a correct sentence and
2)this person was in public schools before 14.
@chang202401 Correct. Until the age of 14, he attended public schools.
@johan did he attend public schools at the age of 14, or did he graduate when he was 14
@chang202401 Yes, the person was in public schools before the age of 14.

The sentence means that the individual attended public schools from an earlier age (not specified in the sentence) and stopped doing so when they reached the age of 14. It implies that up to the age of 14, their education took place in public schools.
@johan got it. So the action of attending can last for years and not necessarily accomplished in one day or several hours.