Locking the numbers in each quarter despite student adds/drops

This will follow a calendar that I will set and while we can go back and look at all of the data, things can not be changed, as we are not able to get any historical data due to the transient nature of the students. at the end of testing we have 25 kids on grade level, 2 weeks later we may have 27 or 19.... there needs to be a way to lock that data each 8 weeks so that the historical does not fluctuate.