I think I would look at the root cause, malicious behaviour. It's really hard to address in a school environment. Perhaps impossible.
Have you got threat protection enabled and set to prefer security? That should help stop students from launching attacks on outside parties.
https://documentation.meraki.com/MX/Content_Filtering_and_Threat_Protection/Threat_Protection
I assume you are using content filtering. Make sure you also filter out things like:
Bot Nets
Illegal
Malware Sites
Proxy avoidance and anonymizers
You'll want to filter other things being a school, but include those above as they are security-related.
The MX does have two WAN interfaces. You could look at attaching an additional separate Internet connection to that and allowing the traffic to be load-balanced across the two connections. Then you could use two public IP addresses. Not a big improvement mind you.