Mark 4: 12 That seeing they may see, and not perceive; and hearing they may hear, and not understand; lest at any time they should be converted, and [their] sins should be forgiven them .
Is this the mark (no pun intended) of a good teacher?
Perhaps you should read the corresponding account in Matthew 13; it clears this issue up quite nicely.
Matthew 13: 10-18
10The disciples came to him and asked, "Why do you speak to the people in parables?"
11He replied, "The knowledge of the secrets of the kingdom of heaven has been given to you, but not to them. 12Whoever has will be given more, and he will have an abundance. Whoever does not have, even what he has will be taken from him. 13This is why I speak to them in parables:
"Though seeing, they do not see;
though hearing, they do not hear or understand.
14In them is fulfilled the prophecy of Isaiah:
" 'You will be ever hearing but never understanding;
you will be ever seeing but never perceiving.
15For this people's heart has become calloused;
they hardly hear with their ears,
and they have closed their eyes.
Otherwise they might see with their eyes,
hear with their ears,
understand with their hearts
and turn, and I would heal them.'[Isaiah 6:9,10]
16But blessed are your eyes because they see, and your ears because they hear. 17For I tell you the truth, many prophets and righteous men longed to see what you see but did not see it, and to hear what you hear but did not hear it.
18"Listen then to what the parable of the sower means . . .
What this passage shows is that the while Jesus said everything in parables to the crowd, the meaning of those parables (aka the 'secrets ot the kingdom of heaven' from verse 11) was revealed only to the disciples. Why? Because, as Jesus indicates, the disciples possess a certain quality that the rest of the crowd lacks. Of the crowd Jesus says,
For this people's heart has become calloused;
they hardly hear with their ears,
and they have closed their eyes.
But to the disciples he says:
But blessed are your eyes because they see, and your ears because they hear.
In other words, the disciples were willing and ready to hear and accept the truth. This attitude qualified them for deeper knowledge of the truth about God. In contrast, the crowd having already closed their minds, were now in danger of altogether losing the the little they knew. This is the meaning of Jesus' words in verse 12:
Whoever has will be given more, and he will have an abundance. Whoever does not have, even what he has will be taken from him.
So then, what is the mark of a good teacher? I submit to you that it is
the ability to recognise a good student, and invest in him accordingly.