Honestly, I don't think that it is right to ban Spanish during breaks and free time such as between classes, but I can see banning Spanish within classrooms. When I took French, we weren't allowed to speak English during class as speaking French would allow us to have a more immersive experience.
If the school is having problems with their English language skills, then I can see how the principal would react this way. A bit too aggressive in my opinion, but understandable.