The band director would like the band to sound good. The school one won't sound good even if you can play it, and worse when you're just learning.
If money is tight, use the school one for the first year as it is mostly learning to play the notes, read music, follow the beat, etc.
Would be better to invest in some private lessons if child is seriously interested in practicing. But you probably don't need private lessons either for the first 6 months. Child can learn to do scales and practice reading music on their own. (Then you can always learn how to play via other online resources.)
Then if really serious, start taking private lessons.
