of the class / for many people

[puzzle]

He is the tallest boy of the class.
Is it right to use "of" here? Please.


English is used as the second language for many people in many countries.
Is it right to use "for" here? Please.

[Rover_KE]

I'd say 'He is the tallest boy in the class'.

'English is the second language of many people in many countries.'

Rover

[5jj]

Or:

English is used as the second language by many people in many countries.

I think I'd use 'a second language' in that version.

[puzzle]

Or:

English is used as the second language by many people in many countries.

I think I'd use 'a second language' in that version.

Is it totally wrong with using "as the second language" here, or just informal? Please.