Formal:
* Did not pass
* Unsuccessful
* Below passing grade
* Failed to meet requirements
* Did not achieve the necessary standards
Informal:
* Didn't do so well
* Didn't quite make it
* Didn't get a passing grade
* Scored poorly
* Didn't pass the test
More playful:
* Took a tumble
* Didn't quite hit the mark
* Didn't quite make the grade
* Had a bit of a setback
* Didn't quite get there this time
The best euphemism will depend on the situation and the intended audience.