To create a new web application using classic mode, run the following:
New-SPWebApplication -Name "Portal" -ApplicationPool "Classic App Pool" -AuthenticationMethod "Kerberos" -ApplicationPoolAccount"Contoso\SPFarm" -Port 80 -URL "http://portal.contoso.com" -Path "C:\inetpub\wwwroot\wss\VirtualDirectories\Portal" -HostHeader "portal.contoso.com"
If you are (most likely) upgrading a content database from SharePoint 2010, you will need to run the Test-SPContentDatabase / Mount-SPContentDatabase commands afterwards to get your old database associated with the new web application. You cannot attach a 2010 content database and create the web application in Kerberos mode simultaneously.
Be sure to register the farm account as a managed account, and also add your Kerberos entries using SETSPN before running this.