When Did It Become Acceptable?

When did it become acceptable to spell things with a 'z' when you should be using an 's'?
This is half spelling-Nazi motivated and half serious question; is it now common practice to spell things like 'trivialise' or 'specialize' like that? When did this become OK? Has it always been OK and I'm just being a complete speng?
Is that how you spell things in your personal statements, in exams or on your C.V? Have your teachers and lecturers taught you this way or what?

http://www.tysto.com/uk-us-spelling-list.html

I refuse to spell most of those words the american way, they just look weird with a "z"..

It's the American spelling for most of the words. As we more widely consume American media, certain words and phrases and yes, spellings too, fall into our vocabularies.

Probably some time in the 19th century when the OUP adopted Oxford spelling.

https://en.wikipedia.org/wiki/Oxford_spelling

Of this I am well aware, but on the serious side of my question, is it now acceptable to use the American spelling for words on official documents/in exams etc.? If you did that when I was in school you'd get laughed at and have to do it again, correctly.

Well I never! This is fascinating!

My question still stands though. (general) Standards have changed in the past decade or so. This is the only forum I've been on where I see the -ize suffix in widespread use by British people.

Same as wannabe "prom goers" who use the "semester" instead of "term"
They tend to have watched high school musical too. Often

The yanks use semesters, incorrectly as it happens, as a semester is 6 months, not 10 weeks
Technically, if you want to, use trimester.

IMHO item shows laziness not to use UK spellings

When did it become acceptable to spell things with a 'z' when you should be using an 's'?
This is half spelling-Nazi motivated and half serious question; is it now common practice to spell things like 'trivialise' or 'specialize' like that? When did this become OK? Has it always been OK and I'm just being a complete speng?
Is that how you spell things in your personal statements, in exams or on your C.V? Have your teachers and lecturers taught you this way or what?

http://www.tysto.com/uk-us-spelling-list.html