I'm at work/I'm in work.

Please correct the mistakes.

Where are you? I’m at work/I’m in work.
Where are you? I’m at class/I’m in class.

I’m at work.
I’m in class. / I’m at school.

Do you understand why?

‘I’m at work’ and ‘I’m in work’ are both possible, as are ‘I’m at the office’ and ‘I’m in the office’.

I’m in class.

Bev, would you really say “I’m in work”? Sounds very strange to me and certainly doesn’t mean “I’m at work”.[YSaerTTEW443543]

TOEIC listening, talks: Commercial offering special prices on mens’ suits[YSaerTTEW443543]

If speaking about being in the building where you work, it is possible.

Expressions such as this, in a telephone conversation:
I can’t talk to you now, I’m in work.
are quite common.

This is something about which there will be disagreement. It’s like forming a queue. People from Ohio stand IN line, and folks from New Jersey stand ON line.

I would say “at work” and “in class”, but I don’t get too upset when others claim to be in work or at class.