When do I report being arrested?

Immediately.

All arrests, regardless of the issue involved, the location of the arrest, or when it took place MUST be immediately reported in writing to the Office of Personnel Investigations and your Principal. The best way is by e-mail to OPIarrestnotifiction [at] schools [dot] nyc [dot] gov (OPIarrestnotification[at]schools[dot]nyc[dot]gov) and cc your Principal.