housebreaking rescued dog

Question
I just adopted a 9 month old female dog named Charly from a rescue shelter in Cleveland, Ohio.  The dog is thought to be a boxer/lab mix, however there is really no way one can be certain on this.  The past owner of the dog must have gotten very angry with Charly whenever she had an accident in the house, because Charly refuses to go when anyone is around. Bringing her to my apartment in Cincinnati, she held it for about 6 or 7 hours, just to wait for me to leave the room to  pee and poop on my bedroom floor.  I'll take her outside, walk her for atleast 30 minutes at a time, and it never fails that as soon as she is back inside, she'll get behind my back and go.  I am looking for as quick as a fix as possible, seeing that I have classes monday through friday, and she will be alone for atleast 3 hours each day.  I do monitor her food and water and take her out about 40 minutes after she eats.. but this has not helped. I've had her for 2 days.. so far her record is 4 times in the house, once outside.
I figured her old owner scolded her which probably led to the problem, so I just ignore her when i find a new mess and clean it up without her watching.. will that build her trust for me and maybe allow her to go to the bathroom with me around.. and outside!?
(sorry for the rambling, by the way).

Answer
I think the best way to fix this problem is with a crate,and keep her in there all the time except to eat,to be fed,to be taken out ect and keep her in there for about three weeks and slowly let her out more-but still keep her in there when you are out,and dogs keep crates clean as they like there sleeping areas clean

So this is the best way to go,as when you go out she will be in her crate,so will be unable to soil the house-also taking her out every couple of hours of being in the crate will force them to do there bussiness outside