As others have mentioned, it depends entirely on intent and context.
Radar is wrong (once again.)
If there was a history of problems between the students and the teacher, the students were probably trying to settle an old score under the guise of a prank. That would be assault, and jail is justified.
If the students had no history of problems with the teacher, and they were approaching the teacher in a lighthearted friendly manner, I would say it falls in the unfortunate prank category. It's disrespectful, but it's not worthy of a criminal record and time in jail. Suspension would be appropriate in this case.
The teacher's point of view is critical here. We can only speculate.
Another news source
[IMG]
http://images.ibsys.com/2004/0518/3320314_200X150.jpg[/IMG]