There is, today, only one kind of written English (with a slight allowance for a sizeable chunk of its users being taught to write in typo). It is inappropriate that pupils not be taught how to write in standard English and advocacy of chaos to suggest that English return to a time when people would spell egg several different ways. It is disingenuous to suggest that the standardisation of written English has much to do with colonialism when it has everything to do with printing and continental Europe’s lettering (i.e. where a lot of printers came from).
Indeed, it remains the case in England that certain dialects are grammatically, er, divergent when compared to what we might consider conventional English. This is exactly the same problem with the same causes, but is completely ignored in a colonial conception of the issue. I believe in a standard written English and feel that it is possible to have distinct verbal grammars work with the same written system. You might disagree, of course.
This does beg the question of what to do about divergent Englishes. In my view, it is entirely inappropriate to try and standardise expression, particularly verbally… and that very much does have colonial overtures on top of a general push for standardisation. In general, fears of a tidal wave of text speak spellings have failed to eventuate because pupils are more than capable of recognising that there is a standard academic English that is used for academic purposes which differs to other Englishes which they are familiar with. I do not see why we should imagine this knowledge does not apply broadly and hence why it is insufficient to simply not police verbal language.
Naturally, academic English is hardly appropriate for all assessment tasks/purposes and I would say that some of the most rewarding means of presentation are those that allow more expression, e.g. dialogues (I wrote quite a few of these at school, for instance).