@Astronema
He's not dense. He's fully aware that Nagisa and Miku likes him. He just can't do anything about it. He's partial to Miku, so he can't simply accept Nagisa's feeling. He can't accept Miku because she's still underaged and also his student. He rejected the president's confession because he already knew he had fallen for Miku.
He doesn't reject because he's dense. He rejects them because he doesn't want to lead them on. That and also if he accepts Miku, he can lose his job. Miku's parents may have given him their blessing, but the PTA and the alumni will not want to just allow a teacher who 'seduce' a student to remain at school.