123.300 and 123.500 MHz are recommended by FCC.
The frequencies 121.950, 123.300 and 123.500 MHz are available for
assignment to aviation support stations used for pilot training,
coordination of lighter-than-air aircraft operations, or coordination
of soaring or free ballooning activities.
Applicants for 121.950 MHz
must coordinate their ...
FAA in AC 90-50, allocates the related frequencies as follows:
121.950: Aviation Instructional and Support
122.750: Aircraft Air-to-Air
122.775: Aviation Instructional and Support
122.850: Special Use and Aviation Support on Non interference Basis
123.025: Helicopter Air-to-Air
123.300: Aviation Support
123.500: Aviation Support
Glider schools ...