We're definitely missing context here, we completely miss what led up to it.. but I don't see how you can say using mace is justified. It's not some random criminal, it's a teacher, they are identified, they are not going to disappear. It's also on school property. You handle something like this in court, not by assaulting someone.

Doing a little bit of research (although not vetting sources, and just clicking the first results that come up) I seem to find this sentiment about phones and school:

Typically, state law permits the governing body of a school district, like your local school board, to create school policies about the possession and use of cellphones by students on school grounds. Schools take different approaches, which can make it difficult to know for certain what the cellphone policy is at a particular school.

and

If schools have a policy that bans personal electronic devices, including but not limited to cell phones, laptops or iPads, then teachers or administrators are allowed to confiscate these devices if students are using them against the policy.