To set multiple channels using the "SetChannel.exe" utility, simply enclose the channel numbers in curly brackets like this: C:\SetChannel.exe {10,20,450}
There is also a multi-string registry key on the Teacher computer located in Computer\HKLM\SOFTWARE\Wow6432Node\LanSchool\ called "GroupChannels" which defines multiple channels.
Good luck with your deployment of LanSchool!