Concept:
The primary role of a teacher is to promote the overall development of learners, including intellectual, physical, emotional, social, moral and ethical development.
Step 1: Understand the teacher's role.
A teacher not only imparts knowledge but also develops students'
• Intellectual abilities,
• Physical well-being,
• Moral values,
• Social and emotional skills.
Step 2: Analyse the options.
• Raise intellectual standards -- Only one aspect.
• Improve physical development -- Only one aspect.
• Help in all-round development -- Covers every dimension of education. \checkmark
• Inculcate value system -- Important but only one responsibility.
Step 3: Final conclusion.
Hence, the primary responsibility of a teacher is
\[
\boxed{\text{Help in all-round development of the students}.}
\]