Thread:Quandlm/@comment-44586401-20191205073111/@comment-36168319-20191206134848

Thanks for the reply,

The part you're referring to is Sae's words for Suzune about her opinion on why Kiyotaka was "the most defective student", which was also just her personal opinion.

For the reason why he was put into D-Class, there is an explanation in Volume 7 (taken from the fan translation):