So many things! If I could only choose 5 bullet points though, they would be:
Funding
The system is chronically underfunded. Buildings are falling apart, resources are not fit for purpose. Teachers spend their own money on basic things like pens. It's utterly shameful.
Inclusion
Controversial, but we need to stop banging this drum. Yes, of course, we can try to make the learning environment accessible for more children. However, it is always going to be the case that a significant minority of children need specialist or different provision. Failing to provide this fails both those with SEN and those without. (This IMO as a SEN mum, so even more contentious).
Curriculum
It is overful, especially at primary. There are so many irrelevant things we have to teach these children, which are then abandoned at secondary. What is the point? It squeezes time for the more creative subjects out and kills the love of learning. No 10 year old needs to be spending weeks on fronted adverbials and expanded noun phrases. We do not need them to all have identical handwriting, like Victorians. As long as it's legible, who gives a shit? Not secondary schools, or employers...
Discipline
At secondary in particular, the balance of where this lies is all wrong. Whether my son is wearing a jumper or not in hot weather has no bearing on his ability to learn. Is the uniform he is wearing the correct items? If yes, then at 16 he should be able to decide how many layers he wears without asking permission from anybody else. Likewise, it's not rude for him to not respond to a teacher who refuses to pronounce his (perfectly normal) name correctly after teaching him for over a year. It's rude of them not to use his name correctly. I will die on that hill.
Meanwhile, there are kids dealing drugs who get away with a slap on the wrist. Ridiculous. I agree with previous points around over surveillance here too. Your parents would not have known you wore socks with personality to school, but these days there are apps that teachers are made to log every tiny indiscretion on and it's really quite disturbing. I think you get away with more self expression in prison.
Holidays/Timetabling
We do not need 6 weeks in summer, and we can definitely give children more than 20 minutes to eat lunch. My daughter is in primary and only gets 15-20 minutes to eat, she's not allowed longer because they get shuffled off outside and can't take what they haven't eaten yet with them. She's a slow eater. She's hungry.