Exegesis:

ouk estin mathētēs huper ton didaskalon ‘a disciple is not above his teacher.’ The article ton has possessive force. The saying must be understood as having a bearing upon the leadership of the disciples, see below. huper means here ‘surpassing’
and refers to authority and insight. didaskalon is best understood as referring to Jesus.

katērtismenos de pas estai hōs ho didaskalos autou ‘every disciple, who is fully trained will be like his teacher.’ The subject of estai is either ‘he,’ to be understood from and referring back to mathētēs in the preceding clause (with pas going with katērtismenos), or pas, with mathētēs understood, ‘every disciple’ (cf. Revised Standard Version). The latter seems preferable.

katartizō ‘to make complete’; here ‘to train fully.’

Translation:

Is not above, or, ‘is not more/better/greater/higher than,’ in the sense of ‘is not more important than.’

When he is fully taught, i.e. when he has been taught all things he has to know, or, when he has learned everything he has to learn (cf. Kekchi). In some languages one can use a rather technical term for having finished a course, or, graduated from a school (e.g. in Bahasa Indonesia RC).

Will be like here refers to equality in degree of authority or capability; hence, ‘will be as great/high/important as,’ cf. also “will reach his teacher’s level” (New English Bible).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.