this is ds's first time having full-day anything and man is it tough! before this year - he would wake up around 9-9:30 then have afternoon kindergarten. now - he is up at 7 and in bed by 8:00 so that has been hard but he is sooooo tired and rammy as soon as he gets home! its like he's punch drunk - all silly...and the "out of the blue" tantrums! wtf!

i know he'll get used to it but sheesh - its tough now!

Maybe he isn't ready... Not all children at the 'approved' age for 1st grade are ready. Waldorf certainly doesn't think so. Montessori also is laid back (for lack of a better word).
Here in VA, you can hold your child back to start kindergarten a year later. I know your son is in 1st grade now, so that option is sort of passed.
If he didn't meet all the benchmarks for passing kindergarten you have the right to request he repeat it and it certainly would not hurt if there are skills that were not 100% grasped. Or you can pull him out for a year and homeschool and readdress the school hours next year.
Hugs... It is tough to have melt downs with 6 yr olds, 2yr olds it is expected at 6 it isn't, so it hits you hard. It is also super tough to go from 9am wake-up to 7am wake-up -- I don't like it myself!!! But man it is nice to have some quiet in the house with the oldest away in school, my oldest is 2nd grade.
Since you wrote this a few days ago, are things getting better?

I just wanted to pop in and say that dd started full day kindergarten this year and was having some difficulty with her attitude and it was totally from lack of sleep (ok maybe a little because of such a big change in her life too lol) But she was getting up at 7 and being put to bed around 8 or 8:30, wouldnt fall asleep until 9 or later (much later some nights). Ive changed the kids bedtime to between 7 and 7:30, usually 7:30 and she falls asleep by 8 most of the time. She is doing much better now. She wakes up refreshed and well rested and is in a VERY cooperative mood. She comes home from school and is just...her normal self and not all busy body trying to stay moving to stay awake kwim? Anyhow, my kids were so adjusted to going to bed at 8 or 9 that I thought that this wouldnt work, but it did and thank goodness!

good luck!