Woah, definitely bad form. You should always ask a person's permission before listing them as a reference. It's not that hard to make a phone call and ask.
Since it's already done, I think you should make your email to your teacher as polite and apologetic as possible, or better yet, call them and explain the situation.
Apologise profusely for not asking their permission- after all, you're putting them out by making them craft some reference bull or talk to the employer over the phone. Don't be afraid to explain that your mum persuaded you to list them, and say that if they would rather not be listed, you can send another email to your future employer and remove them.
I would also start looking for another referee- just in case your teacher refuses/doesn't have time.
Edit: depending on how long ago you sent the CV in, you may be able to pretend to ask permission quickly before the employers ring. It depends on how quickly your teacher replies, and hopefully they will say yes, but that's pretty much your best bet. Otherwise, just be very apologetic.