I still say it and hear it all the time.
You could also say (all very/reasonably colloquial):
It is/was bucketing it down.
It is/was coming down in buckets.
It is/was coming down in sheets.
And then there's this (vulgar)
It's pissing it down.
I've always liked The heavens (have) opened but that is quite specific, it tends to refer to it starting to rain, rather than just being raining.
I'm sure others can come up with some more options and variants too.
- For Teachers