For college financial aid purpose, “custodial parent” means the parent with whom you lived the most during the past 12 months, regardless who has the legal custodial right.
While the Federal government (and most state governments) does not consider the income and assets of the non-custodial parent, many private colleges do consider the non-custodial parent as a potential source of support, and require a supplemental financial aid form from the non-custodial parent.
http://www.finaid.org/questions/divorce.phtml
Private prep (boarding) schools also generally consider both of the parents’ income to calculate parent contribution.
https://www.nais.org/Articles/Documents/Dev_Div_Sep_FA_Policy.pdf