If I could pick one aspect of charter schools that enables them to better serve students, it would be the flexibility to select and employ their own teachers. Charter schools in Colorado do not participate in their authorizing school district's collective bargaining agreement with the teacher's union. Charter school contracts stipulate very clearly that the charter school's employees are employees of the charter school and not the school district. Charter school employment contracts are called "At-Will Agreements" and outline the at-will nature of employment.
For the charter school this means they can employ the very best teachers. Teachers often cite the freedom to manage their own classroom, support from administration with student discipline and the ability to actually teach students as the primary reasons they chose a charter school over a traditional district-operated school. For almost all the people who work in successful charter schools, they're passionate about the opportunity to make a difference in children's lives. Their work is more than a job or paycheck.
Only 45 of the 178 school districts in Colorado have a collective bargaining agreement. Almost all of the charter schools in Colorado have requested and were granted the waiver from negotiating with the teacher's union. In fact, the charter school started by the teacher's union even has the waiver.