When running a Lab on students' PCs, VirtualBox reserves a set of surprises. These are mainly due to hardware compatibility or to OS versions. Here is a non-exhaustive list (hopefully not modified next year) of the problems.
When the error message VT-x/AMD-V hardware acceleration is not available on your system welcomes you, you should be ready to go into the BIOS. The way to do it depends on the hardware. Here is one example for Lenovo Laptops: https://support.lenovo.com/us/en/solutions/ht500216
Go figure out why some PCs did not show a Host Only adaptor. You can type ipconfig to verify that the interface is present. If this is not the case, go to File→Preferences→Network and add a host only interface.
If you are using Host Only interfaces, you should preferably deactivate any external connection such as WiFi or Ethernet. You can also assign static addresses to the host and the guest interfaces in order to enable communications.