Must I inform my teacher that the information I mentioned to him about musical instruments during middle school was incorrect?

What has been narrated in this regard is weak, including the hadith (definition): "Whoever listens to the sound of singing, he will not be permitted to hear the spiritual beings." The prohibition of singing is established by other proofs from the Qur'an and Sunnah (definition). You are not obliged to inform the teacher of this, but do not report anything unless you are certain of its authenticity.