Board Thread:General Discussion/@comment-5719892-20170504181411/@comment-4525039-20170523164014

Taiyou is a first year like Tenma.

To quote from wikipedia, "san" is used as a title of respect at any age, it is the most basic and common form of honorific.

"Senpai" are more directed toward elder colleages or an older grade at school.

Tenma refer to Shindou and the other older members as senpai because Shindou and the others are older than them in the school. Shindou is also more experienced, so it's polite to refer him as Shindou-senpai for the sake of politeness.

While Taiyou is around the same age as Tenma, it's possible that he refers Shindou and the others with "san" is because he sees them as equals plus he is from Arakumo Gakuen and not from Raimon, so he doesn't have the "senpai-kouhai" relationship that Tenma has with Shindou, so he feels that "san" is respectful enough.