Even for a Grade 4 piece, this arrangement seems quite challenging for a young band. With enough time and slow rehearsal, most of the trouble spots can probably be managed. One of the biggest factors is that every part is demanding like a 1st part. The 3rd Clarinets have to play just as high as the firsts (up to an Eb6), the 2nd/3rd trumpets must all play to an F#5 and all trombones are expected to reach G4.
For the brass players, there are a few rhythmic and technical challenges to go with the moderate range issues, but the bulk of the work falls to the woodwinds in this piece. There are frequent sixteenth-note runs including some rather large intervallic leaps. Much of this is thinly scored, as well, making it even more necessary for each player to be solid on their instrument and their part. The oboe has a duet, with the alto solo, that is not doubled or cued in any other part. This whole passage is unison and could, therefore, be left out but it adds a very cool color/texture to the piece and should be maintained if possible.
The amount of mixed-meter and the less familiar meters themselves (5/8 and 7/8) could also prove to be abnormally demanding for a young ensemble
The arranger also calls for great dynamic dexterity, writing from pianissimo to fortissimo. Throughout the work the call for dynamic contrast is great and meeting this demand is essential to effective performance.
Articulations will be challenging. There are many, fast triplet passages with varying articulations. These passages are not always idiomatic, either, giving the players even more to deal with - they will have to figure out the best fingering and manage the changing articulations. If practiced slowly, individually and as a group, these passages can be accomplished successfully but it will take a high effort from each player.
This is a very cool piece and is sure to be a favorite of musicians and audience alike. If you have a dedicated and talented young group and an outstanding saxophonist (almost certainly this will need to be a guest), this could be a very fun and rewarding piece to include on an end-of-the-year concert.