Hi K.C.~
I had the same feeling as you did a few years back when my twins were in first grade. One of their teachers said this to the 4 of us during October conferences (kids and hubby): "We want to see Michael and Christina enter the first grade hallway on their own starting next semester. This new routine will allow them to grow independently."
Well.... it was tough for me to let them go and spread their wings, but it happened! My husband would see them off in the morning too. So the 4 of us walked to school each day and entered the office doors together. We then walked to where the 1st grade hallway BEGINS and my husband and I said our goodbyes from there and watched our kids enter their hallway without us. They got what they needed to get from their lockers and entered their rooms. (Maybe they'll let you transition like this? That helped us.)
STAGE 2 of the TRANSITION....Then when second grade came we weaned ourselves a bit more. We said our goodbyes from just inside the school near the front doors (office doors).
STAGE 3: After a month of this, we stayed right outside the school and said our good byes.
STAGE 4:Third grade came and we said our good byes from 100 ft. away from the front doors.
STAGE 5: Now it's fourth grade and we say our good byes just a bit farther away. (Many kids at this age are saying their goodbyes at home and then walking together or riding bikes to school together.) However my husband and I prefer to still watch to make sure that they enter the school doors. We know they are safe hands when they enter there, too.
So.... yes it was hard but it was worth it. My daughter would cling to me in first grade and keep giving me "just one more kiss and hug." My kids were in the afternoon kindergarten and the teacher just opened the side door and only the kids could come on in. Then when we all entered the front door in 1st grade, we just started a routine of walking them all the way to their classrooms. I liked saying hi to the other kids and to the teacher. I never told long winded stories to the teachers because I knew they were busy.
BUT their were a few moms who would do this and that put the teachers in an awkward position. They probably wanted to tell them to stop talking but they didn't. It also makes the hallways much more crowded and really it DID MAKE MY KIDS FEEL MORE CONFIDENT WITHOUT ME HOVERING AROUND THEM. That was hard to initially take in and accept.... kind of bittersweet, you know?
I would think twice about writing a letter. Your kids will do just fine. It's all about "letting go a bit" and feeling okay with it. Take some deep breaths... I've been in your shoes. You say "they claim it teaches independence." Well... they are RIGHT it does teach each child independence.
I taught 2nd grade before having my kids and all parents said their goodbyes at the recess doors. Then I would come outside when the morning bell rang and I would go to my line of students and walk only the kids inside and upstairs and to their/my classroom.
Good luck.... there are plenty of other things that you can still enjoy doing with your son. Just let this one thing go.