Telegram

Pros
Three types of messages: text, voice, video
Audio- and videocalls
Groups (all) and Channels (one)
Group and individual chats
STICKERS
Switching-off / on of comments
Sharing photos and videos in good quality
Sharing variety of documents (jpg, pm3, mp4, pdf, doc, etc.)
Great number of languages
Synchronization on several devices
Cons
File size limit (BUT the biggest limit among the others, up to 2GB)
No stories
No status
Checking one’s contacts

Use as a resource for learning
Channels with plenty of materials (almost any kind)
The page with selected messages
Feedback through the individual chat or comments

Use as a resource for teaching
A great variety of files to share (posts with images, tables, videos, etc.)
Fixed messages for rules or stuff like that
Channel for materials and group for feedback and questions
OR
Channel for materials and open comments