[asoneth]

I cannot overstate how difficult it is to do this solo. Being the only person in a friend group who does not have a phone results in pretty severe ostracization, especially for preteens and teens who do not yet have a strong sense of self. It means being left out of conversations, invitations, discussions, jokes, parties, homework questions, etc.

Having said that, you are early enough that you still have time to convince the other parents in their grade cohort to take a pledge like "Wait Until 8th" https://www.waituntil8th.org/ (In the US, 8th grade is 13-14 years old.) This provides a kind of "herd immunity" as long as you get enough families on board.

And if your school does not already have them in place you can advocate for anti-cell-phone policies which minimize classroom time as a potential vector.

Be forewarned that you may not be successful unless you are also willing to cut social ties with families who cave in and give their kids phones before then.