The fact that they weren't in uniform doesn't negate the severity of the act. Schools often hold students accountable for their behavior even when off school property if that behavior negatively impacts the school environment or involves other students. The location of the fight (on school property) makes it even more likely that expulsion would be considered.
However, the exact punishment will depend on factors such as:
* Severity of the fight: Was it a minor scuffle or a serious assault?
* Prior disciplinary record: Does the student have a history of behavioral problems?
* School policies: What does the school's code of conduct say about fighting and off-campus behavior?
* State laws: Some states have stricter laws regarding school discipline than others.
* Circumstances surrounding the fight: Was it self-defense? Was it provoked?
In short, while not guaranteed, expulsion is a definite possibility. The student is likely to face significant disciplinary consequences.