An umbrella school is a private school that allows parents to educate their children at home.
There are two exceptions to compulsory attendance at a public school in Colorado: homeschool and private school. Each of those exceptions comes with certain legal requirements, with the private school requirements being slightly less restrictive than those for homeschool.
If you choose to enroll your child in an umbrella school, the child is technically a private school student and reports to the school rather than the public school district and must meet the requirements of the school rather than the homeschool law.