W. Harold Mare, 'Teacher and Rabbi in the New Testament Period' in Grace Theological Journal 11.3 (1970) 11-21, notes the following technical usage of didaskalos in the New Testament:
"That the terms rabbi and didaskalos are understood in the Gospels as equivalents is seen John 1:38 and John 20:16. The complex of rabbi-didaskalos and mathetes (disciple, learner), that is, the master-teacher and his group of followers, is presented regarding Jesus and His disciples in John 1:37-38; 4:31; 9:2; 11:8, and also of John the Baptist and his group (John 3:26)."
After examining the extra-biblical evidence he concludes as follows:
"In summary, it is to be observed that rabbi together with didaskalos began to be used for the idea of teacher-master at about the time of Christ, as is evidenced by the New Testament Gospels and some early archaeological evidence from inscriptions, and the corroborative evidence from Josephus and Philo in the use of equivalent terms. Then as the transition between the Jewish economy and Christian Church continued, the term rabbi no longer had a place in the latter as is evidenced by the lack of the use of the term rabbi in the New Testament outside of the Gospels. Even didaskalos outside the Gospels is sparingly used in the Acts and the Epistles, this latter term seeming to be reserved basically for Jesus (compare also Ignatius; Mag. IX, Jesus Christ, our only didaskalos). This is corroborated in the Apostolic Fathers where rabbi doesn't occur at all and where didaskalos is used but relatively infrequently. But on the other hand, as Judaism continued and developed in its own way, the title “Rabbi” became increasingly important in Jewish practice and tradition as is evidenced by Talmudic tradition."