What is the ruling on students standing for the teacher when he enters the classroom?

The Fatwa's Summary:

It is not permissible for students to stand up for a teacher upon his entry, as this is contrary to the Sunnah of the Prophet, peace and blessings be upon him, based on what was narrated from him: "Whoever loves that men stand up for him, let him take his seat in the Fire." The Companions did not stand up for the Prophet, peace and blessings be upon him, because of his dislike for it. It is not permissible for the teacher to command standing, and it is disliked for the students to stand, due to the violation of the Sunnah involved therein.