echo "Student Id Class
John 23 2
Mark 33 3
Jerry 44 2
Sam 55 2
Tom 65 3
" | perl -ane '$F[2]=~/^\d+$/ or next;push @{$h[$F[2]]},$F[0]}{ $h[$_] && print qq|Class $_ = |,join(",",@{$h[$_]}),qq|\n| for 0..$#h'

##</code><code>##

Class 2 = John,Jerry,Sam
Class 3 = Mark,Tom