Rat spaying question...

We adopted a female rat from the shelter that was supposed to have
been spayed as a lab rat for a (medical or veterinary?) school. She does
have a scar on her underside.
  We put her in with our un-neutered male and they would show no
signs of any mating activity until lately. The male is now mounting her
and trying to mate, At first she's squeak and fight him off, but
yesterday I noticed she seemed to flatten and slightly raise her
hind-quarters and seem to accommodate him.(??)
  So I'm wondering what is going on. When a class uses rats to
practice spaying, do they always accomplish the task? And why would it
have taken a couple months for this activity to begin? Please post or
e-mail us direct ANY information.
Thanks.