but DS6 is in a public "Montessori" classroom, so his teacher is his teacher for 1.2.3
That does make it phenomenally worse!

In that case, I'd try to get a teacher change. Talk to the parents of children in higher grades about what they thought of the different teachers. If you can any find parents of gifted children, that's even better.
Do kids stay and play after school on the playground? This could offer you the opportunity to strike up a conversation with another parent. This is how I found out that DD's 1st grade teacher would be the best match for 1st grade (and probably the best match I'll ever have at this school).
Even with a gem of a teacher who has been receptive to some of things I've advocated for, I've been second guessing whether I should have gone with my original plan to homeschool this year.
The teacher lets my daughter work on the challenging math after completing daily class assignment in the 1st grade workbook. She's also started giving her books to read that are at the appropriate level and more challenging spelling. Even with these accommodations, I know DD is spending much of the day reviewing things she's already mastered.
Knowing that I do have the alternative to homeschool helps keep the panicky and hopeless feelings somewhat in check. This allows me to be a more effective advocate. That's why I'm glad there are people on this board who've made public school work and people who've made homeschooling work. Unfortunately, either way it's a lot of work!