Who are the lambs in John 21:15? At a recent bible study with some lay missionaries, the lay sister leading the study indicated that the lambs in John 21:15 represent the apostles.  She was unable to explain why we might think that to be the case, though.  Is there a tradition or other source of support for the idea that the lambs in this verse represent the apostles?

I'm specifically interested in her claim, not the verse itself, and it the claim's relation to traditional Catholic interpretation of this passage.  I do not care what you think the passage means or why.  I do not care if the interpretation presented is reasonable.  I do not care what the connotations of the word used in the Septuagint or Jerome's Vulgate are, except inasmuch as they represent Church teaching as regards the claim.

I do care what the Church Fathers, both Eastern and Western, said about the verse inasmuch as it relates to Peter's relationship with the apostles.  I do care about Papal bulls, encyclicals, etc that assert the same claim with or without support.  I care about documents giving biblical support for the Papacy which cite this verse or conspicuously do not cite the verse with support for that being conspicuous. 

In sum, I am interested in knowing what support, if any, exists for this specific claim (i.e. that the Catholic Church teaches that in this passage the 'lambs' are the apostles) but not other claims about this verse in general.
The "lambs/sheep" in this case referred to the whole Church

From one of the notes at the Vatican's web site issue of the New American Bible, for John 21:15 we find the following analysis of verses 15-17 in notes 8 &amp; 9: 


  8 [15-23] This section constitutes Peter's rehabilitation and emphasizes his role in the church.


The lambs are addressed in this note:  


  9 [15-17] In these three verses there is a remarkable variety of synonyms: 
  two different Greek verbs for love (see the note on ⇒ John 15:13 {agape/philios}; two verbs for feed/tend; two nouns for sheep; two verbs for know. But apparently there is no difference of meaning. The threefold confession of Peter is meant to counteract his earlier threefold denial (⇒ John 18:17, ⇒ 25, ⇒ 27). The First Vatican Council cited these verses in defining that Jesus after his resurrection gave Peter the jurisdiction of supreme shepherd and ruler over the whole flock.  


A citation(excerpt) from the First Vatican Council:

On the Institution of the Apostolic Primacy in Blessed Peter (Fourth Session, Chapter 1)


  And it was to Peter alone that Jesus, after his resurrection, confided the jurisdiction of supreme pastor and ruler of his whole fold, saying: Feed my lambs, feed my sheep [44]{44. Jn 21, 15-17}. To this absolutely manifest teaching of the sacred scriptures, as it has always been understood by the catholic church, are clearly opposed the distorted opinions of those who misrepresent the form of government which Christ the lord established in his church and deny that Peter, in preference to the rest of the apostles, taken singly or collectively, was endowed by Christ with a true and proper primacy of jurisdiction.  


About lay led retreats and bible studies

It is in my experience common for Catholic lay leaders to have OK, albeit incomplete, explanations for a variety of doctrine and theology.  I have experienced this first hand serving on just under a dozen ACTS retreats.  The enthusiasm of the leaders and team members is often matched by a lack of depth in the grasp of doctrine and various high level Church positions.  

Sources used to supplement lay led bible studies vary widely, from articles in faith based magazine, to the Catechism, notes from formal classes in theology or religion, notes used to teach CCD to children, to favorite blogs, to official doctrine and of course Scripture.  I'll offer the suggestion that the lay leader was passing on something they had learned informally.  For a lay led event's purposes it's close enough not to matter.  The original apostles were very much Jesus' sheep, and if not The Whole Flock they represent a very important flock, or a key part of his flock, whom he sent to the rest for our salvation.