Telegram is an end-to-end messaging and file sharing app. The good choices for instant, private communication are very few. The best ones have proven historically difficult to learn and use. Telegram is a middle of the road good choice.
BASIC PROS AND CONS Cons: You have to trust the team that runs it that they are telling the truth when they say they are not sharing your data, and are encrypting the data; the code is not open source; you do not control the data fully.
Pros: It is very secure and the data is not owned by an American corporation; it was created who seem to have an existential reason to deliver on their promise; it has many useful functions for group work like file and link sharing and search; you can export chat histories; you can delete your own words on both ends.
You can create a Telegram group for student teams, if you have team assignments. The group will be private to them, and they can use the app for ongoing discussion, using voice or text. They can also use the same group to share files and links. Telegram remembers everything that has been shared (unless you erase it) in the app and makes browsing and sharing links and files easy.
Telegram can also be used for the whole class as a class forum. Same as above, but a larger group. Like any other closed class forum, you can set up assignments that involve student responses that take place in the forum.
How to use it
Telegram works on Windows, MacOS and Linux. The software can be easily installed in any one of these platforms. Once installed you can start adding contacts by using their phone numbers. That means in practice you know the person you are adding. Once contacts are added it's relatively intuitive to start forming groups, and start chatting. The system in that sense works similar to WhatsApp.
Facebook Groups, Google Groups, Facebook Messenger, and most other chat solutions.