My school uses the same exact thing to do this to us, what ive figured out with random playing as they dont use it much. right before your teacher does it try moving the mouse as much as possible sometimes this will crash it for some reason and allow you to do whatever, try ctrl+alt+del and start taskmanager and kill the lanschool process. What i did was deleted there lanschool from the computers with an admin account i found laying around
what i did
1.) tried to start cmd but it was disabled by admin, so i got around that by thinking of powershell which was a problem because they had removed it from being able to see and search for that program so what i did is created a batch file with one line "powershell" in it and it worked o.O
2.) I located the lanschool files on the C:\ drive because my school blocked the C: drive after kids where deleting system32 *facepalm*
3.) i deleted the files and walked away from the computer like nothing happend, little did i know they had a backup vnc software that i was able to disable on certain accounts with that admin account i found earlier.
This is highly non-recomended, not only can you get into a lot of trouble but you can mess something up, also im not liable for anything that you do with the knowledge above.