Will this student even be back from the DAEP before school is basically over? You may not need to actually do anything. We had a senior once who was basically gone until the end of the school year because of admittance to DAEP. You could prohibit said student from participating in the spring concert or whatever else is going on simply due to the fact that so much rehearsal in preparation for X activity or concert. If admin is not supporting your decision there are other ways. If this kid is a complete behavioral problem for you as well you should be keeping a log of the indiscretions so that in injunction with the DEAP there is a trail of evidence to show that dismissal from band was in this kid’s future.