Just a marching horn? This may sound crazy, but if you are just looking for a marching band trombone, check out the new Jiggs pBone - plastic trombone. There are plenty of YouTube videos showing these and believe it or not, they sound pretty darn good; and would do a fine job for marching band. And best of all, their price!
Re: yamaha trombone? 08:06 on Sunday, March 04, 2012
Now I have no bone to pick with any of it and am quite a bit older now, but I recently bought a p-bone to practice with becasue of my arthritis (It is so light I can hold it for hours without any pain). So I can tell you for certain that a P-bone is not nearly as out of the realm as you make it sound.
That said, Marching band instruments don't have to be 'great' and indeed, I wouldn't want to take my nicer horns out in the rain and snow, so I'm not inclined to buy a really nice horn for that. Truthfully, you can get by with a Yamaha or King entry level, but if you want a tad more usefulness than just marching, you might want to look at some of the intermediate horns with an F-Attachment like a Yamaha 446G.
I told my friend's son to just keep playing his Yamaha YSL-354 in marching band - it is functional and if something happens and it gets dinged in the stand or rained on for hours...no big deal.