Create a poll in your channel to take attendance:
Go to your channel and type "/poll" into your message box and press enter twice.
Create an "open vote" poll with a "single choice". This will allow you to see who voted on which response, and ensure that they can only vote once.
Allow your students to vote on the poll to let you know they're present at the moment within your channel or sub-channel.
Closing a poll
Finish, or close, the poll after an allotted amount of time. Do this by clicking the three vertical dots to the upper right of the poll itself. Clicking "Finish Poll" will close the polling options so that nobody else can vote, and will also display a timestamp of when the poll was closed.
Check the names beneath each of the polling options to check attendance.