Answer:
This report is not reliable because:
A real increase could be proved with the sample of all the students who attended the study session, due to mustn't be many students (I believe the students regularly doesn't like the extra study sessions), where wouldn't just have honors students, but the regular students too, who probably would actually need increasing their math scores.
Explanation:
To identify if a students population actually increased their math tests scores by a particular reason, the researcher should take a sample from the regular or bad students that attended to the extra session, or at least an equal sample with the same amount of good students and regular students ( or bad students), in this form he would compare the results in each individual and identify if it actually depended on the extra study sessions or from other reasons how study time in the house, learning disorders or others.