The accuracy of this information can be verified by a review of the students' school transcripts. And the surveyor would be able to ascertain if nine out of ten students really got the grade they stated in their survey.
Other options are not relevant. A survey of students from nearby high schools is not relevant to the results of the high school under question, unless the aim is to include the entire district or comparison of schools in a district.
A consult with students' parents may not be very useful, as it may be likely that some parents do not have an accurate idea of their wards' performance in a particular subject.
Since the survey is for this batch of students and the aim is not the comparison with other batches, an interview with previous graduating batches will not help the researchers.
Hope this helps.