Much as we like to be of help by answering questions here, sometimes the best way we can help members is by advising them to use the dictionary. Have you looked these words up? Well, I'll do the work for you now, but please note that if you do it by yourself next time, you will discover how effective it can be.
(4) respect is a noun, and we need an adjective there, because we have the copulative verb be.
It'd be a different story if we had the verb show or have. Students have or show respect for their teachers (or, at least, they should).
(1) respectable is used to refer to someone who deserves respect. If we say that a teacher is respectable, he/she is worthy of respect (students respect or should respect him/her).
(3) respected is a participle and will be used in the passive voice, in sentences like:
- Teachers are usually respected by their students.
The word you want is then (2) respectful, meaning "showing respect."