Frankly, and this is my personal opinion, 4 year olds are not well suited to confined table work; doubly so if the child has known the material being covered since he was 2 (or younger)! I think so many preschools do children a disservice by forcing pen and paper desk work. The situation is even more unfortunate when otherwise creative, inquisitive children are forced to regress in academic skills to conform to a classroom routine.
Does your son enjoy the program otherwise?
You say the preschool is play-based--perhaps it's worth a quick drop-in meeting with the teacher to explain that your son has permission to be exempt from worksheets.
