They don't have quite the same meaning. In fact, if the teacher had said 'up to now' he will have a specific complaint to air with the parent about the child's learning.
'Now' is implying that something has happened, whereas 'so far' expresses a time up to the meeting in which the child's good learning has continued and is not necessarily going to come to an end. Even in this situtation, the teacher is reminding father and and son that good work must continue!
By the way, I would choose the word 'well' instead of 'good'.
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