I think I'm in trouble with my books!!!?

I dunno how long ago it was but my original math book must have gotten switched up with someone elses! When they assigned my math book I used to take it to class all the time because the other books were really beaten up and pages were torn out. I must have gotten mine's switched up with another book that must have been in okay condition because when I opened my book (We hardly ever get math homework and when I opened it we hadn't gotten homework since the beginning of the year) my name wasn't there! The book wasn't assigned to anyone and I still have a couple of months before I have to turn in my book so what should I do?!?! Even though those books are like 10 years old I don't know how much it would cost. If I ask about how much it costs they will be sure something's up so I just need help! I'm too young for a job cuz im only in the 7th grade so that's outta the picture. Forge the sig? But that would be lying and way too difficult to execute- i'd hafta remember the teacher's handwriting and I can't because they switched teachers in the middle of the year! Is there any way I can get out of it without having to pay a bunch of money? I'd be SUPER lucky if they were getting new books but I can't wait for that! If I search the room I'll get more questions than answers so that's not gonna help either. Any ideas?