Also, this just came up [1], and they talk about its use in teaching, though I dont know if it gives you anything beyond the apps students already have?
I'm familiar with Slack so I'll check out Zulip, it looks great!
VoIP is new to me, I will look at that. I also found that Microsoft Teams has policy settings to limit bitrates for specific users, so that might be also an option to limit bandwidth for students with data limits (between 3-6Gb/month in basic plans around here).
There may even be a good-enough auto voice->text transcriber for Portuguese?
A double bonus would be having the teachers review the recording/transcribing -- as some teachers (if not all of us), improve after checking recordings of what we do
Blessings on your efforts
--
I was sitting beside a very impressive Ecuadorean teacher today, with refugees
Sometimes larger change is need for society. Something like having older children teach younger children has been effective in recent history
My most effective technique to-date, has been to pray
Then chat apps such as email or messaging systems are especially great, because when the network drops, nothing is lost and the conversation can resume later
I would suggest teachers do this, possibly with telegram channels or similar. That way the class can have a group to participate in and share problems. Could this be the kind of confrencing you're after. It is as low bandwidth as it gets, resilient against network failure, and allows real time chat in a group...
Often more directivity is wanted though.... One option might be teachers recording audio-visual videos (their voice + preferably blank background slideshow as the visual). Which I think you're already doing
Another option is voip. Which is already heavily optimized for low bandwidth. Something like [1] or the team chat options computer gamers use. Onething to consider is if it is possible to record the audio session for later download -- since it is frustrating for students who can't get in reception range or if the audio drops out -- but a download might let them catch what they missed. If that would be useful to you, I don't know. But I suggest browsing the voip options, since there are many
I'm a programmer. But none of my suggestions involve programming, or creating new apps. There are good reasons for this. 1) It is a lot of work. 2) Students would need to download and learn them. 3) Existing apps they know fix some of these problems really well
One final suggestion. Try zoom, with video disabled, and the shared screen view for teacher to draw on. If this doesn't work for you, then probably no custom app is going to either
[1] https://en.m.wikipedia.org/wiki/Mumble_(software)