I would encourage the students look for shell, and to sketch them in place. Not picking them up, just sketch them as they rest on the sand...even if the water tickles your feet.
Original watercolour painting shell study III by SeasideStudiosUK
organic form <-- I love sketchbook pages like this, full of studies and notes.
23 creative sketchbook examples to inspire high school Painting / Fine Art students
Beaumont School: GCSE Art and Design
Julia Wright Jewellery, mussel shell drawing
Artist Research Page, Francoise Nielly | Flickr - Photo Sharing!
Original Conch Shell sketch on old book page by Yoopershop, $25.00