How to disable access to virtual classroom prior to webinar's start?

In the room turn on the Block Incoming Attendees function (Meeting > Manage Access and Entry > Block Incoming Attendees). You can then put up a custom message telling them that they need to come back at a specified time in the future. This essentially locks the door to the room, but as the host of the room, you can come and go as you please. Then 15 min, or whatever number you want, prior to the start of the class you can turn that function off. Anyone waiting at that screen at the time that you turn it off will be let into the room, as well as anyone joining after you turn it off.