(A teacher can see all the VMs of a class. A student can only see the VMs to which it is assigned.)
In case of Windows instances, you can connect by using VNC or an RDP client. The Windows username is 'Admin' and the password is 'admin'. The first time you log in, you will be prompted to change your password.
In case of Linux instances, you can connect by using the built in WebSSH or an SSH client. In cas of Ubuntu instances, the username is 'ubuntu' and in case of Debian instances, the username is 'debian'. For Linux instances you must use provided private key.