Many counties in Pennsylvania, including Delaware, Bucks, Chester and Montgomery counties, have established August 1 as the deadline for you to appeal the following year's real estate assessment. The assessment is what is used to calculate your property tax bills; if the assessment is too high, you will pay too much.
Accordingly, August 1, 2013 is the deadline for filing an appeal of your 2014 taxes in many counties.
As I have mentioned before, you should not appeal your taxes unless you are confident that your assessment is too high. In many cases, properties are actually underassessed. Often, it is new construction, or houses built in the last few years, that are overassessed.
Since you probably just received your school tax bill, take a look at the assessed value that is listed on that bill. Multiply the assessment by the common level ratio for your county. (For 2014, it is 9.09 for Bucks County, 1.66 for Chester County, 1.35 for Delaware County and 1.58 for Montgomery County). If the number seems to be more than you could get if you were to sell your property, you should consider an assessment appeal - either contact an attorney who handles assessment appeals, or else go the the Board of Assessment Appeals for your county for the forms and further details.