She messed up by slapping him without him hitting her first (unless she claims she hit him while he charged at her; I wasn't clear on whether she stated she hit him while he charged at her or after he went up to her, then stopped). She loses there, and she should be punished for it. I'm also wondering if it's the school's policy for students to wear their pants properly (most likely, it is), as that will determine whether she had a good reason to tell him to fix his pants in the first place.
However......I believe the teacher's account is more accurate. I seriously doubt she would have slapped him after he already respectfully answered her and picked up his pants.
The most likely scenario was that he was being disrespectful after being asked to pick up his pants, then got mad at her and tried to play alpha male against her by getting in her face. She tried to one-up him, and it backfired. She should have let him carry on like a jackass, or even better, let him get the first hit.
Even if the student believed she had no right to tell him to pull up his pants, that doesn't excuse him from being an ass. Of course, most people on here will just focus on the teacher's error and not the student's. These are the same people who somehow find joy and humor in reading about a student disrespecting a teacher....then will complain about young men today not taking school seriously (and will make excuses for their own disobedient kids in the future when they act up in school). How do you expect them to take their education seriously if you excuse them from taking teachers seriously?