Responding to a few comments and questions:
I meant to put in the OP, DSS' bus is only 1/2 full at any given time. I don't know how full the other routes are though, so I would like a resolution that will work for all students, if there truly is a space issue on some buses.
The school has a very limited supply of instruments. These instruments are available to families in need, for rent at a rate that is significantly less than a private rental. Due to our finances, we did look into renting through the school rather than a private company, and there are no trombones available at all. So having a school instrument and home instrument isn't an option on a couple different levels.
In 6th grade the students are supposed to be practicing 1/2 an hour every day (in addition to their class time). If the students only bring their instrument home on the weekends, they end up trying to fit 3 1/2 hours of practice into 2 days, followed by a 5 day break. I'm not sure that this schedule would really lend itself to any level of proficiency, although it is of course better than no practice at all.
DSS' lunch break is 22 minutes, followed by a 22 minute recess. With only 44 minutes break total, there really isn't time for him to practice, stand in the lunch line, and eat. I guess he could practice for 22 minutes during recess, but I think that he may be finishing lunch during recess some days already, if the lunch line is particularly long or slow. I know that a NY school provided a late bus and practice space at the school for students who wanted to practice after school. That seems like a lot of extra expense in bussing though, so I don't know if that would be a reasonable alternative for DSS' school or district.