I would say there are few points that should be taken into account of the free online classroom software.
1. The “stability” ( the quality of the image and sound etc)
2. some functions, such as
screen sharing (available on all of the above),
interactive whiteboard (only available on zoom)
recording (only available on zoom).
3. whether you have an account and download the software in advance ,
Skype : you have to create an account and download the software in advance
Zoom : you do not have to create an account and download the software in advance, instead you will be invited in the classroom via email
The stability of zoom is higher than skype and google hangout since it is a newer technology.
Below extracted from this review : https://www.groovehq.com/blog/zoom-tips-and-tricks
Recording is a tremendously useful feature in Zoom.
Want to keep records of key meetings ? Record them and send them to the person so that he/she can refer back to them.
Just type ⌘ Cmd+⇧ Shift+R (PC: Alt+R) to start recording any meeting.
Type ⌘ Cmd+⇧ Shift+P (PC: Alt+P) to pause/resume recording.