With doozzoo you have an integrated filesharing & cloud storaging .
The invite-link makes it possible to connect the coach and student securely.
E.g. with dropbox you have to invite other persons to a shared folder, too.
And on Skype or zoom, you don’t have this cloud storage functions.
(For a test-session you only have to click "create room" an send the room-link to your test-partner. But in the test-modus, you can't share files and it's limited in time)