The Web App is very intuitive and there are many ways to sign out a student. Please see the animation below for two quick ways to do this for a single student.
A) From the tile
B) From the 'Hover Menu'
Tap on the student's name, then select the 'Leave' button
Select 'New Leave'
Select the leave type
Once you have filled in the form and saved it, the student will be signed out. This will change the appearance on the Home Board as seen here.