We have exactly the same issue, Predominantly when students are cutting and pasting from the internet into Word/Powerpoint. Up to 30 at a time start to freeze. All they can do is pull the plug and reboot. Sometimes they can log straight into the same session, other times it is a new log in. We have HP thin clients and WYSE Winterms both connected to Citrix, occasionally happens with the Winterms but no-where near as much