FIrst of all , go to the school and discuss the desk - that needs to stop NOW. He needs to be included with the class. HE is being labeled and ostracized by the children due to what the teacher has done!!
Is he on an IEP/504 plan? If he isn't, then you need to put in writing to request an evaluation for learning disabilities. Please ask any questions about this.
Also his meds, what has he tried? Did you try to increase the dose before changing? Sometimes it is just the dose, but it could be the med also.
The emotions occur when the meds wear off is called rebound, we need to discuss this. It is as if he is being dropped hard on the floor, all of a sudden, no warning. It is very difficult for these little guys.
Who is his prescribing doctor? We use a psychopharmacologist who specializes with these meds.
I think his meds need to be worked on. Is the medadate successful for the hours he is covered? If so, then can you add a second boost - have the school nurse give it to him.
You know these stories break my heart because I went through similar school crap with my adhd son. He was NEVER excluded with his desk in the classroom however. That is disgusting!! But we did have social exclusion.
Contact his school principal AND t eacher. You need a meeting ASAP. Also
Special education should be brought in to help him. WE also need to help him with his social skills. Do you know if they have social classes at your school, some do! Another question to ask when you call them!! 
Do you think there is a co-morbid as well? My son has anxiety and a speech delay. He also has sensory integration. So he had speech therapy and an OT in elementary school. He was also on a IEP. Now in 7th grade he is on a 504 plan and has speech.
Please let us help you both! 
I agree with Bethann that his meds need to be addressed. (Apparently so did the doc!
) My first thought when you said the Metadate was wearing off in 4 hours was--- if it's is working well for him during the 4 hours then just give him another dose. It sounds like he just metabolizes the medication very fast. My son also metabolizes VERY fast so he takes more doses per day than many other kids. I would just hate for you to miss a medicine that is "right" for him just because it isn't lasting long enough. It's usually an easy fix. You may want to ask the doc about this before switching to Strattera.
I just mention this because we just switched from Ritalin to Stattera and it's a difficult transition if he has to go off of the Metidate completely. The reason is that Strattera doesn't really take effect for weeks. So it may be good to leave him on the Metadate along with it. Something to ask the doc about anyway...
I also agree with Bethann that Strattera isn't as good as the stimulants for hyperactivity or impulsiveness. We ended up adding some Ritalin back in for my son (at a much lower dose than he was on) and this, along with the Strattera, seems to be working well. But the Strattera took about 6 weeks to really take effect.
It's good that you are going to talk to the school! It breaks my heart to read about the desk being against the wall!
Go set 'em straight mom!
Good luck and let us know how it goes.