Here is a contingency plan:

Have the child have a "text" cell phone.  If lockdown ever occurs, they need to ring  you which classroom they are in.  Tape the tablets to a brick and have someone throw them in there through the window---through glass if necessary.   I guess this all depends ifhow much open area is where the classroom is really though.  At Ash’s school some classrooms face the sports feild, so there is no way to take cover whilst you leg a brick in there.  LOL!    Get the child to give the phone in charge in that room so that they can take cover from broken glass.  Well...this is the only thing I can come up with, and it is the only practical solution in that scenario.  I’d be damned if I would not even have them try it in the circumstances really.  Get to know the classroom numbers in the school and what windows are relative to them.    ;)