Why is Yukihira Soma not famous at Tōtsuki Culinary Academy?
I have not read the entire manga, but I read that Soma's father was formerly known as Saiba Jōichirō. It is not the same last name as Yukihira Soma, so this may be a reason why people don't know that they are father and son.

Source: Jōichirō Yukihira on Shokugeki no Souma wiki.