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)