Hi dmercer,  I agree with you that this sounds like an incredibly harsh punishment for any child, much less an entire class.  More importantly, it's likely to be incredibly ineffective and result in creating a lot more problems than it solves.  Kids definitley need breaks during their day (we all do!) and boys especially need opportunities to be physically active during the day.  As these kids sit in silence for 7 hours a day they're going to be building up resentment against the teacher (which sets up a really bad situation for the rest of the year), and against the kid who originally made the fresh comment (which really isn't fair to him).  They'll lose ground in their studies and fall behind in the curriculum.  Most of all, they could develop a really negative attitude about school that could stay with them for the rest of their lives.

I really think you need to ask for a meeting with the teacher TODAY to learn more about what's going on.  (There may be more to the story than you've heard from your son).  If you're not happy after that meeting then I think you should go directly to the headmasters office to meet with him or her.  

If they both agree that the punishment should stand, then you may have to get creative and think of other options.  (Could you homeschool your son for these four weeks?  Are there other parents who are upset who may support your efforts to change the punishement?).  This is not as much about what's "Legal" ...it's about what's best for your son.  I agree with you that four weeks of silent punishment really isn't best.

Please let me know what happens.

Oh - and I'm reading a book now called Raising Cain that's really great and actually talks a lot about the negative consequences of harsh punishments for boys.  You might pick it up and offer it to the teacher or headmaster to help with your conversations.

Good luck!

Boys need their active time.  It actually helps them in class.  Contact the principle of that grade, 1st then the headmaster.  My son attends a private school.  They would only have to sit out one day.  They have to have a lunch break, I believe this is legally. They can have a silent lunch though.  Which at my sons private school is only one day.  How long has she been there? Possiblly too long.  And time to retire.  I had a problem the teacher was unreasonable.  The principal of that grade and I met.  And then I met with the rest of his teachers in a conference.  Can you get any other parents involved in speaking with the principal? The parents of the other kids would help.  My son is now in the 6th grade. Hope this helps let me know..