In Commonwealth v. Goslin, the court concluded:

Contrary to the trial court's conclusion, the “other lawful purpose” language does not restrict the defense provided in Section 912(c). Instead, the phrase does just the opposite: it expands the defense to include any additional or different lawful reason not otherwise mentioned in the first clause of Section 912(c), regardless of whether it is school-related. To conclude otherwise, would make “possessed for other lawful purpose” redundant with “possessed and used in association with a lawful supervised school activity or course.”

The Superior Court's holding in Goslin is very broad. It means that if the defense can produce evidence of a lawful purpose for which the weapon was possessed, the prosecution likely should not be able to obtain a conviction under the statute.

Is the reason that you carried the firearm on school property is for self protection or the protection of students from a illegal school shooter?

It would seem that the court says this is a valid legal purpose.