Objective: The comparison of baseline neuropsychological test results with post-concussion neuropsychological test results is a common practice to assess the aftermath of a sport-related concussion. The effectiveness of this approach is compromised when invalid baseline test performances occur. The present study was designed to assess the prevalence of invalid baseline test results of high school athletes, using the Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT). Method: A large sample of 8938 high school athletes’ baseline test data were examined, including the four ImPACT Composite scores and three sets of embedded invalid indices: five ImPACT Invalidity Indicators, four ImPACT sandbagging red flags, and two ImPACT sandbagging subtest flags. Results: There were 8394 (93.91%) Valid scorers, who failed no Invalidity Indicators; 544 (6.09%) Invalid scorers, who failed at least one Invalidity Indicator, 2718 (30.41%) Red Flag scorers, who had at least one Red Flag score; and 4154 (46.47%) Subtest Flag scorers, who had at least one Subtest Flag score. Of the entire sample, 4485 (54.65%) failed at least one of the invalid indices. Conclusions: As in prior studies of high school athletes, the Invalidity Indicators identified a low portion (6.09%) of the athletes with invalid baseline test results, while other invalidity measures found 30.41% to 46.48% of the athletes producing questionable test results. The high rate of invalid test findings suggested in this research calls for greater efforts to improve the accuracy of baseline test results.