There are multiple ways to do this, but this is the easiest way to show what is on your computer on the TV (and vice versa). 

First, on your TV, click on the InstaQShare app. On the right side of the screen, you will your TV's name, it may currently be named after the teacher in the room or the room number. 

On your computer you should have a program installed named InstaShare or InstaShare_BENQ   . Open it up, then find that same name that is listed on your TV from the previous step. Click on the TV's name, and click connect. Next, click Start Casting. You can now see and control your computer through the TV. You can also do the opposite (see and control your TV from your computer) by clicking Screen Mirror.