Most high schools have either/both the ACT or SAT averages by year available on their web-site.
Something else to consider, and it may depend on whether you live in the Midwest versus a coast, is what percentage of students took a particular test. I think a general rule of thumb is that most high school students take the predominant test for their area, but the higher scoring kids tend to take the predominant test and the secondary test for the area which could skew results for the less usual one.