The funny thing is is that all the examples you gave are correct English.
[I learned it] "at school" - I learned it at school, I'm not specifying which school
[I learned it] "at the school" - I learned it at school, the one that was referenced
"I know the English" - I know the English people
"I know the English language" - I know the English language (specifically)
The point being that articles do a lot of work. They're not just decoration whereby there is some arbitrary correct form or rule. It's a matter of what you want to say.
But in the third and forth sentences 'English' is both a collective noun and the name of a language which is just a coincidence.
[I learned it] "at school" - I learned it at school, I'm not specifying which school
[I learned it] "at the school" - I learned it at school, the one that was referenced
"I know the English" - I know the English people
"I know the English language" - I know the English language (specifically)
The point being that articles do a lot of work. They're not just decoration whereby there is some arbitrary correct form or rule. It's a matter of what you want to say.
But in the third and forth sentences 'English' is both a collective noun and the name of a language which is just a coincidence.