Another vote for open ended play at this age (my girls are almost 3 1/2) unless the child is asking for "school". World book has a list of what kids are "supposed" to learn at each grade level. It begins with preschool.
http://www.worldbook.com/wb/Students?curriculum Here's my theory on how to use this "tool". Look at what age your child will be when you start teaching "kindergarten" (for me, this is 6. Our state requires kindergarten at age 5, but from what I hear, it's quite easy to get a one year extension on that age, so I'll do that. Partially because if I were putting my kids in a classroom environment I'd hold them out until they were 6 (my mom held both my brother & I out a year , I started K when I was 6, my brother started 1st grade when he was 7 (the school we were attending when he started school didn't offer K) and I think it was a very wise decision
), partially because I'm "lazy" and figure there's no reason to deal w/ reporting to the state for an extra year if I don't have to), then the year before that is the year to make sure they have a relatively good knowledge of the stuff listed on the preschool list. So . . . basically when they're 5 1/2 or thereabouts I need to look over the preschool list & see if there are areas I need to encourage thier interests in. I know when I looked over the list originally my kids could already do many of the things, other than those that are tied into fine motor skills & such that my kids don't have yet. They'll grow/change a LOT between age 3 & age 5 so I'm certainly not going to worry NOW about the fact that they can't write their letters, they're not developmentally at a place of having the hand-eye control to do that, it's getting there, they're starting more & more to draw "pictures" instead of scribbles, their own idea. Those pictures are starting to occasionally be recognizable. I would expect that before they're 6 they'll be asking me how to draw letters (Ashlyn's already asking when she'll be able to read, so I wouldn't be suprised if, in the next year, she either learns to read herself (as we were walking through a store today one of the girls (I'm SO bad about remembering who says what after the fact LOL) totally randomly (granted, I hadn't been listening as they talked to each other, so may have missed the "context") said "puh, pig . . . Mommy, does pig start w/ P?") or will just figure it out on their own.
So . . . I'd say certainly until they're 4 or 5, let them play, answer their questions, provide resources for things they're interested in, including structured "school" if they show an interest, but otherwise, there's plenty of time for "school" once they're older, if you decide to go that route.