There's several books that include essays by, or interviews with, former students of his: C. S. Lewis at the Breakfast Table, We Remember C. S. Lewis, and In Search of C. S. Lewis are three I have that are quite informative.
Plus, some of the biographies of his students have some interest in this regard: Pack My Bag: A Self-Portrait, Golden String, Sprightly Running, and Young Betjeman. Interesting how many of his students have had biographies written about them.
One thing to note, Lewis didn't teach seminars or courses the way most of us think of them. His instruction was eithier weekly tutoring sessions with two students at a time (where he would recommend readings, assign a paper to be written, and listen to the paper assigned the previous week) or lectures to groups.