When first connecting the iPad to your display, the iPad will mirror the desktop to the external display. As soon as you open the Virtual app, the mirroring will stop and the screen go black, ready for a virtual video class to start

In the unlikely event the virtual app is not mirrored on your display, unplug the white Apple Digital AV adapter and reinsert. If the problem persists, check you are using a genuine Apple Digital AV adapter. 

Non-genuine adapters are NOT SUPPORTED and may cause reliability and connection issues.