Hello, I hope you are doing well

In this kind of situation, the best option for you is to go to your school admin office and explain them the whole situation that you have lost your hall ticket by mistake.

Best approach to do this would be write an application regarding the issue, to the officials of your school. Most probably, they will help you and provide you another admit card.

But if they don't help, then you have only one option which is to write the application to your board officials or you can also visit the regional office of the examination board you are concerned about. You may have to pay some amount for it. And make sure that you have other documents with you as they will require it to produce new admit card.

Hope this helps