One of my research projects right now involves my 4th great grandmother's 2nd husband. I was doing a survey of what resources exist in various nearby libraries for the area that they lived in and was pleased to find a school census list for their county. A school census can be important for a few reasons. One important aspect of the school census is it provides a snapshot of your ancestor and their family at a certain place at a certain time. It allows you to know where they lived and the names of the children in the family.
Now in my case, I was able to look at a transcription of a school census that showed the name of the parent (mostly the names of the fathers), how many kids they had, kid's ages and what area of the county they lived in. Because it was a transcription, I need to now look to the state archives to research the original records, hopefully on microfilm, to glean any information that may not have been part of the transcription.
School census records may be in a number of places. They may be transcribed in a book, or available from the Family History Library on microfilm, or through a state archive/library. And if your ancestor was away at school during a federal census year, they will appear with their classmates on the U.S. Federal Census. This might be the case for your ancestor if they went to boarding school, lived at college or were at an Indian school.
A unexpected surprise for me as I researched the school census was that my 4th great grandmother's 1st husband (for whom she divorced) and her 2nd husband appear to live near, if not next door, to each other. This provides some additional information to help better understand the dynamics going on with these two families.