"Hardass" for trying to protect her offspring from problems like this? You're just lucky it wasn't a real bust and that you were only caught by mom! She is being responsible and doing only what most of the rest of the parents would do if they found contraband in their kids' room.

You're doing all you can do right now...that is giving up toking and concentrating on your classes. You won't be able to change her mind in the short months you have left so just kick back and give in to the house rules for a little while longer. It will be far easier than living under an umbrella of distrust and lies and be all that much more enjoyable when you can toke up without the paranoia.