Though graded a 3 by the publisher, some of the woodwind and trumpet lines may be more difficult. I would give this a 3+ due to some of the challenging articulated passages, the 2nd clarinets' being asked to play consistently around the break and the range of the 1st clarinets (up to E6). The trumpets do not have to play exceedingly high (only to a G5) but there are some fairly technical, articulated passages that could be a concern. A few key, exposed oboe lines could require rewriting, if no suitable oboists is available, as the lines are not doubled or cued anywhere in the ensemble.
The dynamic contrast written in the piece is plentiful, but the real challenge will be achieving that contrast while maintaining a balance with the soloist. This is always tricky, especially for young musicians, but the issue is exacerbated in this work due to the soloist's range and timbre. Whatever instrument plays the solo it will be low-pitched and not easily projected. Making sure this remains the predominant voice will be critical to success.
Between sections there are little mini-cadenzas for the soloists. Getting in and out of these may take some work and will take careful watching by the ensemble. Each occurrence ends with a fermata but there are no caesuras; each is a continuation directly to the next phrase. There are also a number of Rubato sections, for the soloist, which are not easy to line up; the soloist is free to take liberties (generally slowing down as it is marked "lazy") and the ensemble must come back in at tempo with little preparation.
Though there are some challenges, this is a great novelty piece to feature a strong low brass or low reed player and it is definitely worth the time investment to give these musicians, so often neglected in young band music, a chance to shine. There are enough ways that this could be adapted/arranged, that the technical demands should not prevent a young band from playing this. Some proficiency for sure is needed, but this piece is a manageable, worthy endeavor for the middle school band.