Another solution is that the child arrives slightly late to madrassa to change. Ask the child if he doesn't mind changing into Shalwar Kameez for the madrassa instead of going commando on the Imam.
psyah, I DO NOT HAVE ANY PROBLEM WITH GETTING HIM CHANGED BEFORE GOING TO MADARSA. IN FACT I DID THAT COUPLE OF TIMES IN EARLY DAYS OF THE CLASSES. Problem is the wrong concept that my child is learning i.e Islam has "something" to do with Shilwar Qameez.
As far as respect of teacher is concerned, no denying that but please for heaven sake, you are not required to obey all wrong orders too specially if they are adding "bad-dat" to Islam. If they make it a dress code, I'll love to oblige.
For all the "shilwar QAmeez" lovers, let me assure you guys, even though I am out of Pakistan from last 11/12 years, each member of the family has reasonable number of shilwar Qameez from good quality from JJ, Adnan's and other designers to everyday use shilwar qameez. We all love to wear it on occasions so please be over the concept that I have any problem with Shilwaq Qameez.