Have you considered storing materials in your room? 

Well, in my honors ensemble, we have a little tradition. If you come to rehearsal without your music, you have the opportunity to stand up in front of the group and sing "I'm A Little Teapot," complete with gestures, of course. An alum actually decorated a baseball cap for the purpose, so that's become part of the ritual. It's a pretty good deterrent.

I issue detentions for repeat offenders.  The first day without materials gets them a warning, the second is a call home, and then I start issuing detentions.  They are progressive as well, starting with 15 minutes, then 30, then 45, etc.  I have no mercy regarding after school sports, and if they are in a sport, one conversation with Coach usually does the job.  BTW, I consider being prepared to be on the risers with pencil and music when the bell rings, so if they have to go to their backpack on the sidelines to get a pencil , it starts the progressive discipline.  

 

Dont be afraid to set strict boundaries.  Yes, you may occasionally get a parent upset that they have to come get their kid, but a firm reminder that it is the student's responsibility to come to class prepared and that you are teaching a life skill is in order.  Once you start following through with consequences they will immediately shape up.  Students who are held accountable will rise to meet the challenge.

 

Oh, and make sure this policy is in the class syllabus you have parents and students sign at the beginning of the year.