Forgetting homework is a detention first time here too.
My DC's get 6 chances re forgetting a pen etc though.
I think OP means they're "silly" mistakes for her DS to be making, rather than that the school is being silly punishing him for them, right?!
Some children find it harder than others to adapt to the organisational requirements of secondary, but given the right support at home, he will.
DS2 writes homework in his planner on the day it has to be in, as well as the day it's set, so that he can check the planner as he packs his bag and see which HW he needs to take IYSWIM.
DS1, who really struggled to begin with, has decided it's easier just to carry everything all the time, so he's never without the books he needs.
TBH, if I head my Dcs were being disruptive silly in class, detention would be the least of their worries!