Diligent, I'm going to step in to defend teachers. Not all are lazy. In fact, there are some very good teachers out there, who are just as dismayed as others at declining levels of literacy.
The problem lies with those responsible for setting the English curriculum (bureaucrats). During the seventies and eighties there was a move away from rote learning of grammar rules, in favour of the notion that grammar could be learned through usage. Methinks this notion is partially correct: those who never sat parsing sentences are still intelligible, even if they do not use standard grammar correctly. However, the policy was taken too far... it became a case of 'anything goes'.
Certainly, language evolves over time and it makes little sense to force adherence to a set of rules that do not reflect actual usage. For example, why compel people to use 'whom', when in practice it has become obsolete? The meaning is still clear when 'who' is substituted. The focus should be on ensuring that meaning is conveyed accurately. Unfortunately, the policy meant that errors were not corrected, because students would self-correct through exposure and usage.
And yes, spellcheck is to blame. I've seen 'pubic' substituted for 'public' a couple of times. I happily comply with requests to not make information pubic...
