Should do, St Marys College is not as popular as other sites due to its location. When I walk back at night, its is pretty quiet, just be aware of your suroundings (although that applies to everywhere). I always tell people to walk back either through Castle Leazes or follow the 63 bus route, not cut through the council estates or the parks (follow the 36 bus route) and I have had no trouble at all. There seems to be quite a few chav gangs that hang around along the 36 bus route, although no incidents I have heard of.
Its about 40 minutes walk from the centre and 30 minutes walk to the centre, because it is up a hill.