If you are a builder you might say, 'I work on a big school', but if you are a teacher or an employee of a school, then you would write 'I work in a big school.'
If you live in India, you might say, 'I work on a big elephant', but if you are an employee of a big company you would say, 'I work in a big company.' Of course, you could add, 'and they pay me lots and lost of money.' Yes? No?