Kevin Federline never paid up on a bill from his kids' private school, and now the school says he owes over $15,000 ... this according to a new lawsuit.
Britney Spears' ex-husband is being sued by the West Valley Christian School, where K-Fed and his current wife, Victoria, send their 2 daughters.
According to the docs, obtained by TMZ, the prep school claims Kevin and Victoria signed a payment contract back in August 2018 when their kids' started going to West Valley Christian ... but the institution claims the parents haven't paid since May 2019, and are running up a huge balance.
The Christian school claims K-Fed and Victoria now owe $15,593 for daughters Jordan and Peyton's tuition ... claiming the couple "breached the agreement by failing to pay for services which had been provided on credit at their special instance and request."
In the docs, the school says it already demanded the money, but Kevin and Victoria are refusing to pay all or any part of the remaining balance.
The private school wants more than just the $15,593 it says the parents owe -- the school is also seeking 10% interest per year, dating back to May 2019.
Remember, Kevin has 4 other kids ... a daughter and son with ex-fiancée Shar Jackson, plus his 2 boys with Britney.
The lawsuit was filed one week before we first reported on Kevin's plans to move to Hawaii with his Britney's boys, Sean Preston and Jayden James.