"受訪者是中學生,而他們的回覆率很高"
The interviewers (mostly) are high school students, their feedback percentage is pretty high.
Very likely, the word "interviewer" is the right word as '受訪者' and not "participant" (關係者;參與者) 資料 from: English Heritage
The participants of the interview are secondary school students, and their response rate is high.
you can choose to use present tense or past tense here, but I would recommend present tense.
Although it is a past event, it is still true that the participants are secondary school students, and the response rate is high.