I would suggest that it be made VERY clear to the child that the technology in question be used during school hours ONLY for D purposes. Have it put in a 504 that the form of technology is for this reason. Make it clear to the school that you will defend this right as much as possible. However, if the child uses it inappropriately, let the child, and the school, know that you will not tolerate that. Set very clear school usage guidelines.
A cell phone for the purpose of texting BG numbers to the parent is good. We have "blocks" set on our children's phones during school hours. They can ONLY send and receive from us (Verizon Mobile...small monthly fee...can set up online.)
If the child has a smartphone with a D app., as long as they actually use the app for the purposes of D, it should be allowed.