For people wondering why there aren't more exclusions or pupils being sent to PRUs. It's really a combination of factors. Firstly, a school cannot just exclude a pupil with no alternative provision, they have to make arrangements to educate them.
However, the alternative provisions available for violent pupils are limited because a) it used to be that it was only 1 or 2 pupils in a year group who were violent - now it's 20-30. There isn't the alternative provision available for them all. b) because they have to prioritise the very worst "offenders", pupils aren't removed for a very long time and school's requests for a place for a certain pupil are often rejected the first few times (at least). c) it's very, very, very expensive for a school to pay for a pupil to go to a PRU - and that money comes from the budget to fund the education of the other pupils.
So, if they sent 1/6 of the school (all the violent pupils) to the PRU (even in an imaginary world where there are enough places available), it would use up the school's entire budget - they wouldn't be able to pay for anything for any other pupils (no staff, no books, no exams, no heating etc).
Moreover, it's a really long process to remove a child and involves the whole appeals process from the parents too. By the time the whole process has been followed, the pupil will have been in school for years and years. This doesn't even take into account for the added complexities of when the pupil has special consideration (for example if they have SEN or difficulties at home etc).
This is all with reference to my experience in secondary, I don't know anything about primary.