Although there’s no guarantee what the school’s API will be 10-20 years from now, you can have a good estimate, based on it’s past history. Look back over the last 10 years and see if their API has been consistent. Then take a look at the community. Is there going to be a lot of new housings being built in the future? If so, what kind of housing? If you’re in a built out area, the likelihood of the demographic changing over the next 10-20 years is unlikely, especially if you’re in a more high end area.