If you have lost the class 9 registration slip, then at first you have to be sure that its lost. If you are sure then you need to file a complaint about the losing of your registration slip at the nearest police station. With the xerox copy of the complaint you have to visit your school and they will guide you out the forms and other details. This kind of problems are usually solve by the school authorities. You may be charged some charges for a duplicate registration slip, but you will get it. Just contact your school with proper documents.

I hope this answer helps. All the very best for your future endeavors!