Is threatening your children good parenting?

Nononononononononononononononomnononomnomono.

That is not good parenting.

I am calling social services.
Maybe.
Not really, but the point is still there.

Yes, if it produces the desired result.  Your example of going to school was not a threat. It was a statement of what might happen to them. Threatening (or possibly a warning) suggests a course of action to be taken against you which you may find unpleasant. It does not have to be physical. E.G. It could be 'being grounded' or having your allowance stopped.
On the other hand, for parents to allow the continuance of bad behaviour or lack of discipline, without taking appropriate action, would be bad parenting.

Sometimes they do it because it works. My step dad took me to court when i was 13.

Depends on what the threat is.  The threat of consequence is ok but the threat of a beating isn't.