Are there convenient services for educational conferences?

Question

Skype

There is an online course on an educational platform with an individual approach to learning, as a result, there are 15 teachers for one course of about 300 students. It is necessary to organize weekly conferences within groups of approximately 12-20 people for 2 hours. We just used Skype, but the possibility of moderation is very lacking. We found several free services made on WebRTC, but they all connect directly, not through a server, as a result, 6 people - a maximum, then the connection becomes unsuitable for communication.
The corporate account hangouts, iMind and TokBox came up in all respects, but on average it turns out from 20 to 40 tr. per month with such a volume of students, but for us it is too expensive. It needs to be cheaper. What other services can you try?