What to do about preschool.

I have a bit of a dilemma on my hands. My older two children both have spring birthdays so deciding when to sign themup for preschool was a no brainer. They started in the fall after they turned 3. My third and fourth children are both fall babies and their birthdays fall after the cut- off date for our local school district. The third is almost three now and seems interested in starting school, but if he starts now he will have three years of preschool. It would be good socialization wise for him to have other children to play with and to separate from me for a period of time each week, but academically he already knows his letters and numbers, colors and shapes and even some sight words even through there are 3 years until he can enter kindergarten. Also, he isn't fully potty trained which isn't an issue for the one preschool but is for the other. If heis going to start I want him to start on the first day so my planof action for this week is to call the preschool and see if they have any room left and what times the classes are to see if they conflict with the pick-up times I have for my other two children from their bus drop offs.