What an interesting thread. My experience is quite the reverse. Schools will not exclude for behaviours that in any other environment would be regarded as unlawful. At work they would result in dismissal for gross misconduct; in society they would be likely to result in a crimina record. Sometimes the children concerned are already known to the police.
No young person should suffer due to the behaviour of another - physically, mentally or academically. Schools do not sanction unacceptable behaviour enough and I imagine that grades are affected by just continuous low level disruption that impedes thorough teaching and revision of the syllabus. It also has a significant impact on the motivation of the majority and the motivation of the teaching staff.
When my dd attended a state school behaviours that did not result in permanent exclusion were: assault, foul language (frequent at teachers), intimidation, severe disruption, theft, pyromania. After four and a half years (my dd had left the school by then because of the shocking standards) two of the young people were permanently excluded but not before they had been allowed to wreak havoc on the rest of the year group and school community.
The issue really is, in my experience, that Head Teachers have not fought for the rights of the majority to learn or done enough to safeguard the education of the compliant majority. Therefore an excuse culture based on unspeakable low standards of social expectation has sprung up with excuse after excuse being made for the anti-social behaviour of the minority. What of the compliant children from difficult backgrounds who want to work and do work and of whom there are far more than the non-compliant - why can't they be given every opportunity to maximise their potential and they are ones who probably suffer most because they have to watch their step on their home ground to keep themselves under the radar and don't have parents bolstering excellent teaching skills made inadequate due to classroom behaviour with private tutoring. They surely are the ones who need more than any other a safe learning environment where they can thrive and achievement will not be mocked. There is news this week about the failure of white British boys to succeed in education and there is surely a link here.
Ultimately society has to accept that one size doesn't fit all and that different types of schools for young people with different needs are required. Not all schools can fight for places in the league tables but all schools should be focussing on providing every child with the best possible education they are able to access and we as a society need to start making a distinction again between well educated and well qualified.
There needs also, of course, to be far more investment in things like pupil referral units and support and budget for children who cannot peacably cope with mainstream school. One size doesn't fit all; some children would be better elsewhere, somewhere that can meet their needs.
Nothing was done to ensure my dd's inclusion in a small comprehensive where behaviour was running riot and the school was declining and not delivering on promises made at admissions. Nothing was done to ensure her needs were met or that she was happy and thriving. The disruption upset her, the violence upset her, the yelling and shouting and mass detentions upset her. That wasn't inclusive and we voluntarily excluded her and sent her to the independent sector where she thrived and grew in confidence. It wasn't even a particularly academic independent but it had the basics in place and expectations in line with normal, decent behaviour and if a young person there had behaved in the same way as at the old school there would have been no second chances.