For middle schoolers learning about box and whisker plots, here's a great worksheet all about finding the interquartile range of a data set. Follow the link to see an info sheet on how to find lower quartiles. Follow the link to see an info sheet on how to find upper quartiles.
Note: There are two different ways to calculate interquartile range. #1: the upper and lower quartile both include the median. #2: the upper and lower quartile do not include the median. Both ways are correct.