When my DD was 4 I had her signed up for two preK (M/W/F & T/Th sessions) so that she would end up in preK for 2.5 hours for 5 days a week (I figured that would be a good lead in to K). She enjoyed the M/W/F session but wanted to leave the T/Th session (same teachers for both) because of the other kids in there. She said the T/Th kids all knew each other and didn't invite her to play, and that they didn't know their alphabet when she was reading and writing a little. So she ended up just going to preK for 2.5 hours for 3 days a week and had a lot of fun. I too had wanted to gently lead in to 5 days of K (half days) for the next year. But that's not how things turned out.
By the time there were parent-teacher conferences that year, her teachers told me they didn't know what K would teach her. I submitted an application to have her considered for Early Access to 1st grade as a 5 year old, and she was accepted after a series of evaluations.
She started 6.5 hour school days for 5 days a week when she was 5. Yes, she was a bit tired. Yes, she was a bit lost without knowing the various school procedures like any kid who just starts school. No, 1st grade was not even a challenge academically, but it was socially and physically, and the beginning took adjusting for all of us. Now, at the end of 4th grade, I can't imagine her being in 3rd grade. She enjoys school even though it's still too easy, loves her friends, and seems well-adjusted.
The adjustment to full days does take some time and is a "big jump" but I don't think it was damaging.
