Do I have to pay back the child benefit?

Hello, the following situation:

I am 19 years old

I finished my sales training in June and then started my vocational high school diploma in August. However, I dropped out in September because I'm now pursuing a different path. I informed the family benefits office directly, and they stopped the payments. Now the family benefits office wants a refund of €750 (July-September) because entitlement for this period has apparently not been proven, arguing that my vocational training was discontinued. I did attend school during this period, so why do I still have to pay it back? What can I do about it? I still have the opportunity to express my opinion.

Thank you very much for your help!