Prior to constructing the questions, you need to determine the research questions and objectives. Your interview questions will operationalise your research questions and objectives as well.
Notably, for testing and measuring teachers/ students' perceptions, attitudes and feelings about a construct like reflective teaching, the interview is one of the relevant instruments. To develop a carefully-designed interview with appropriate credibility, however, you should follow several steps.
a) Define the construct and identify its underlying sub-domains.
b) Decide on the type of interview: structured, unstructured, or semi-structured.
c) For each sub-domain form a few questions.
d) Consult colleagues and experts about the effectiveness of the questions.
e) Pilot test the interview to see if it satisfies the requirements predicted by the research hypothesis.
In my opinion, you should apply a questionnaire to teachers and students first, and then, perform a focused- group interview by forming a panel discussion to find if the responses to items on the questionnaire are valid. This provides you with a triangulation technique and enhances the dependability of your work.