Luke chapter 06 (a)

40 A student is not above his teacher, but everyone who is fully trained will be like his teacher. 

There is another verse alike in Matthew:

A student is not above his teacher, nor a servant above his master. (Mt 10.24)

These two verses refer to different meanings. The one in Matthew talks about the sufferings the disciples will face. Jesus encouraged the disciples that students' sufferings will not so painful above than their teacher's (Jesus's). So that they (and we) have no need to be afraid. 

However, the other one in Luke talks about respecting each other and the humbleness. It's the same as the parable of ver.39:

39 [...]Can a blind man lead a blind man? Will they not both fall into a pit?

That's to say that no one is better or stronger than others. Everyone has his own blindness and weakness. Students might be weaker than their teacher at the moment, but they eventually become as strong as teachers. So that we should respect each other, and humblely reflect ourselves all the time. As the teaching in Philippians:

Do nothing out of selfish ambition or vain conceit, but in humility consider others better than yourselves. (Ph2.3)

Therefore, whenever we see others in a mockery or a condemnatory way, Jesus teach us to check ourselves first :

 41 Why do you look at the speck of sawdust in your brother's eye and pay no attention to the plank in your own eye? 

These parable led me to think of an interesting example of mine. One of my eye has once become ill of myiodesopsia(飛蚊症 or light flaking off of retina), so that for a long time, there was always one split in my eyesight. In this way, when I saw the world with this illed eye, everything seems unperfect and with a split. However, the problem did't happen on things but on me. In the same way, the speck in other's eye might come from the plank in our own eyesight. Therefore, the best way to take the speck off is to recognize the existence of our own plank, and then take it off.