Translation:Our teacher is not old.
There is always a middle ground between opposite adjectives. Something that is not tall isn't necessary short - it can be normal height. Similarly, this sentence only implies that the teacher is not old. If the teacher is a normal 30-40 yrs old adult, that does not make him young.
Similarly, for the Chinese adjective 不錯, it only means not bad, but it does not mean you are doing good