For the most part, the students "mat" their own work. I cut construction
paper to the appropriate size for the piece and they glue or staple the work
to the mat. For our art show, a parent cuts heavy poster board (railroad
board!) in to frames....with either 8-1/2 x 11-1/2 or 12-1/2 x 16-1/2
openings and then we tape the work to the frame.
I often staple the kinderpaintings to the mat because they warp and are
difficult to glue.