you should try zoom or bluejeans, it's been extremely stable and consistent for us, we do conference calls from behind china's great firewall to palo alto with little to no lag, even has HD video.
We're building a virtual classroom solution and we now always force every user to go through a wizard which checks their audio and video, clearly marks when there is no sound being received and connects them to a support person to help them resolve their AV issues before they can join the meeting.
It's a little annoying for the person who wants to join, but the experience for the people already in the meeting/classroom is much better.
One interesting part is that, just like in a real classroom, by default everybody is unmuted. So you can have discussions with 80 students and one or multiple teachers. This makes stuff like echo cancellation, reducing the noise floor etc. a big part of the problem we attempt to solve.