Your reference to the priest asking those “who are practicing to become Catholic” to leave before Communion begins leads me to think he is addressing this request to those who are currently enrolled in RCIA.
In ancient times, those studying to become Catholic were dismissed from the congregation after the liturgy of the word concluded and before the liturgy of the Eucharist began. The idea was that the Eucharist was only for the fully initiated (i.e., those already Catholic). Catechumens were sent off to study with their teachers. At Easter, once initiated into the Church, the new Catholics would participate in the liturgy of the Eucharist for the first time.
In recent decades, with the establishment of RCIA, some parishes have begun to re-implement this ancient practice, excusing the RCIA members after the Nicene Creed to attend RCIA class with their instructors. Some parishes do this for the entire RCIA term, others limit it to the scrutinies during Lent. Personally, I dislike the idea – partly because those who are seeking to become Catholic should be given plenty of opportunity to learn the liturgy and to practice participating in Mass before becoming Catholic, and partly because RCIA often includes not just unbaptized catechumens but baptized candidates and adult Catholic confirmation candidates who belong at Mass.
Nevertheless, if your parish uses this ritual, keep in mind that the intention is to prepare the RCIA class for entry into the Church; there is no intention to “exclude” anyone from the Mass.
