State law [C.R.S. 22-32-109.4] requires the publication of districts with or without collective bargaining agreements with teacher's unions. I count 133 of the state's 178 districts do NOT have collective bargaining agreements.
The largest school district without a collective bargaining agreement is the Academy 20 school district in El Paso County. The smallest district with a collective bargaining agreement is the Centennial R-1 school district in south central Colorado. Coincidentally, Academy 20 also has the largest brick-and-mortar charter school in Colorado: The Classical Academy with about 2300 students.