I can answer this for the high school I attended and from which my sister just graduated.  Some of it seems normal; other things my school did totally differently.

-  kids caught buying drugs are not expelled, maximum punishment is 2 days suspension   - Kids are not expelled, but the suspension would likely be longer than two days.

- regular attainment of 35% test and exam marks is considered "good performance" (not for the child, but for any pupil) - Nope

- problems of behaviour, exam performance, detentions, violations of school rules etc are all omitted from parent evening discussions as "they focus on the negative too much" and it would be "distressing for the kids" to hear that in front of their parents - It might not be the focus of regularly scheduled parent-teacher conferences, but the parents would be summoned to the school for a meeting.  The school can't make the parents attend, but they'd try.

- detentions are not seen as a serious issue or something to be appalled at getting, by school, parents or pupils - This is true.  People aren't appalled.  They're certainly not a good thing, though!

- children can claim to have disabilities and difficulties with no proof and they get extra time for tests and exams and can complete exams using the faster typing on computers - Nope.  No extra time without an IEP or 504, which requires proof.

- grammar is virtually completely ignored when marking homework, courswork, tests and exams - Grammar and spelling are not a big focus in many classes

- registration is supposed to be compulsory, but children turn up half an hour later on a daily basis and the school doesn't do or say anything. - Our school would do something.  Students are marked late, and a certain number of unexcused tardies is equal to an unexcused absence.  Parents would be notified.

I have been a teacher for 26 years before I quit at age 46 so I am afraid to say that is the new norm.

Parents make huge sacrifices to live in the really good school districts.  No one wants their kids to go to school with kids that don't show up or don't care. 

I would seriously doubt this is a "high performing school" (outstanding?) but you used "apparently" so I would let it slide. With the school letting this kind of behaviour slide, I would look at their last Ofsted performance review - and if the school has a funky review not in line with  what you know, I would raise a formal complaint through Ofsted.

https://reports.ofsted.gov.uk/ You can check the school's inspection reports accross a range of categories here, and what the school needs to pay attention to.

Also look at the school's own policies. I just checked my son's school's online website and his school has 40... everything from admissions, to uniform, to sport codes, to drugs, to absences, to food, to admissions to A-levels...

At my son's school - according to the policies - your complaints about absences, drugs, bad behaviour, detentions, etc., if persistent will have parents called in and possibility lead to permanent exclusion (nice way to say expulsion). The instance of kids with learning disabilities - at my son's school - it is an area that is rated as "good", not "outstanding". The last finding the school had was to make further accommodations for children with learning disabilities with support to be supplied to those teachers. (At present. none of these are applicable to my son - yet. My personal experience of transgressions at the school is as a result of my SS, and I  know they don't mess about in trying to protect children or the reputation of the school if need be. My xH and I were called in numerous times around SS's antics at school and he was threatened with expulsion in his A-level year. It knocked some sense into him.)

The school results for GCSE's is outstanding but A-levels are only good. School was told that their teachers need to be consistent in expectations of students, improve their mathematics and foreign language teaching, as well as put more financial resources into teaching kids with disabilities - the last school inspection was mid 2018. You can check if the school's reports are consistent with what you know.

Please check their last Ofsted report - something is definitely not right here. Or the school doesn't care, in which case I would seriously recommend changing schools besides reading your s/kid the riot act if this behaviour is applicable to him.