Cut a medium-sized slit across the closed end of a large plastic trash bag. If using a sheet, cut a medium-sized slit near the very center of the sheet.
If the bag or sheet is too large, cut the bottom so that it does not go past the student's waist to allow his or her legs and arms to move more freely.
If using a sheet, the extra material cut from the bottom of the sheet can be used to make a hood. Cut the material into the shape of large half-circle and sew or staple its diameter to one side of the slit at the top of the poncho.