I thought minimum effort was correct, but someone just told me minimal effort can also be used. Could someone please verify this?
* Not a teacher
Yes, it can be used. I have just checked:
COCA: 75 hits
BNC: 10 hits
Google Book Corpus: 4,544 hits (only 2000s)
"Minimal effort" is a common expression. It means very little effort is needed.
"Minimum effort" is not as common and means something different. It means that the absolute least amount of effort was used.
The boys were able to push the car out of the snowbank with minimal effort.
The boy was in danger of failing math class because he only put in the minimum effort.