How would I look up case law dealing with child support modification after the child begins college?

Some states have provisions for payment of child support after age 18 or graduation from high school for purposes of attending college and some don't. This website will tell you which ones do and which don't....Depending on what state you live in, you may not be entitled to child support once the "child" is in college. In any case, it doesn't really have anything to do with how much time is being spent with the custodial parent....

http://www.dadsamerica.org/cs18.htm

Case law has fewer ramifications in civil law than for criminal law. It's pretty much a case-by-case basis regarding the circumstances in that situation. Extended child support (for the SUPPORT of the child, duh!) as a student usually applies to children living at home. In other words, children still in high school after the age of 18. Unless the custodial parent is paying for college, there is little justification for the continuation of support for a non-resident child, even under the claim of 'student'.

If the child does happen to still be living with the custodial parent while attending college, there are too many issues to establish precedent. One is any obligation to pay for the education of an adult child, and that depends upon ability. Another is any condition or ability of the adult child to work or otherwise help pay for upkeep.

There are just too many variables to establish any firm precedents. While there is some obligation to provide college education for a child, it's not absolute.