I've been saying this on MN for years! We really do need to stop schools and teaching being dictated according to age and the automatic moving up year by year whether the pupil is ready or not, whether academically or behaviourally.
Obviously not a permanent "holding back" year after year, but maybe just an automatic right for a one year hold back, with maybe a second time for exceptional circumstances.
It's a good time to do it now with the forecast reducing numbers in schools over the next few years.
If we can't do that, then there needs to be more of "mixing it up" in the same year, for pupils who aren't progressing, either due to being around "bad influences" or other reasons. I.e. form swapping, class swapping, etc. Do it at the first signs of problems such as disruption, bullying, etc., before the issues get too serious and formal disciplines/suspensions etc are needed.
If a pupil starts secondary in a "bad" environment of a form, i.e. being bullied, too many disruptive pupils around them, they're basically stuck there for 5 years and that can kill their education stone dead. However, if it was easier to shake up a bit, swap forms, etc., nip it in the bud, and the kids who want to learn and grow have a better chance.