Routine, routine, routine.
I swear it’s the only way. Make it a routine to repack the school bag with the right stuff for whatever subjects she has the following day, the night before.
Make it a routine to check said bag the next morning.
Make it a routine to check every room as she leaves it for anything she might have left behind. (Do this at home too so it becomes engrained)
Rhyme off the essentials she needs to remember (what you would put on a laminated sheet) and make it a routine to rhyme them off in her head each time she moves to a new room/end of each period/end of lunch etc.
My DD is 11 and in the last year of primary but this has been ongoing for years with her - I think we’ve finally cracked it and I do believe she will be fine at secondary because the routines have become so engrained now.
For her, the essentials she rhymes off are:
glasses, pencil case, water bottle, lunch.
bag, sweater, coat.
She says it in that order to herself, in a kind of rhythmic way - like with a beat to it, and mentally ticks them off to ensure she has them all (or doesn’t if for instance she had school dinners instead of a packed lunch that day) Because the rhythm stays the same she doesn’t accidentally miss things out.