Adobe Connect

I think you mean Adobe Connect, as Adobe Presenter is not used for live lessons.

Can you have mulitple people speaking in an Adobe Connect room at the same time without getting screeching feedback? Yes. But, you will need to either work with your users to get the correct setup on the hardware or make some changes in Connect.

In Connect, you can go to the following menu, Meeting > Preferences > Microphone. Select the Use Speex Codec in this meeting room and put the quality at whatever you want. This audio codec does eliminate the feed back, but does so by only allowing 'he who speeks loudest' to be heard.

If you don't want to go that route, then you are looking at needing to set some rules for your speakers on how to stop this problem. The problem comes from someone having a mic able to hear the computer speakers. Generally this is due to someone using a built in mic, webcam or free standing mic with their computer speakers turned on. The easiest solution is to have all speakers use headsets. This will mean the mic will never hear the audio coming out of the speakers. If your speakers are strongly against wearing a headset, then they can look at using a USB Speakerphone (easily found on Amazon) which has built in, harware based audio duplexing (like a phone with speaker phone functionality) and this should eliminate the feed back. Good quality headsets run $40-100 depending on features, but more than $100 and you won't get any additional benefit as far as Connect is concerned. USB Speaker phones tend to be more expensive, but sometimes you can find deals.

The other solution is to have the speakers mute their mic when not talking and mute their speakers when talking, but this is a lot of work and usually not worth all the extra clicks.