Agressive behaviour

hi
We rescued a young (11months old) female rottweiler/beauceron cross
bred, about two months ago from our local dog pound. We chose her
because at the pound when all mayhem was breaking out she stayed very
calm. She is very gentle and is great with our young children (1 year
and two years). However we have a problem with aggressive behaviour
towards strangers in the street and other strange dogs(particularly
those who are barking behind fences) ( to the point of biting one of
our neighbours we met in the road in front of our house at the end of
a walk!.
I try to restrain her and make her sit ....she has none of it and will
pull and jump on the leash.
If she is off the leash she will run up to the person barking and
snarling (and already one bite)
We have started dog training classes but when she is there she is very
well behaved (typical!) and the trainers have a problem visualising
this aggressive behaviour. During the training wee do a routine where
we meet other people and dogs (socialisation), she is very calm when
we do this!
Maybe the behaviour is a protective/territorial reaction...the sweet
little old lady we met (and terrified) in the street yesterday was
hardly a threat...
We live in a small village however I'm very reluctant to let her run
off the leash in case we see some one else. How can I correct this
behaviour?