ClassLink Roster Server builds upon the OneRoster specification from IMS Global. ClassLink can provide vendors access to rosters via a REST API or CSV files for any district that uses the ClassLink Roster Server. We do not charge any fees to access the OneRoster data. Yes, it's really free and the school district has full control over the data.

The OneRoster™ standard is a subset of the Learning Information Services standard that focuses on the school’s needs to exchange roster information and grades. The standard includes both SOAP and REST-based bindings to make it quicker and easier to implement the exchange of information about people, membership, courses and outcomes. In addition to the standard, OneRoster™ includes a format for CSV files that are typically exchanged between the school and the vendor to populate the roster information needed to gain access to learning tools, portals and learning environments.

To view more information on the OneRoster specification, go to the IMS Global website.
http://www.oneroster.org