From the school's perspective, that's likely still considered a success. Lawyers are creative, and school systems (as an extension of the government) are rich targets.
If someone is naive enough to use the school communication system for those things and the monitoring software gets triggered and intervention occurred, then the school has documented proof that they did what they could. If something uses the school system and the monitoring software missed signs of it, then it's the software's fault and not theirs. If someone is shrewd enough to not use the school's system for that because they know it's being monitored, well then you can't blame the school for not seeing messages it had no access to.
Only one of those three scenarios does much to prevent things from happening, and is the least likely one. But all three of the scenarios limit the potential to somehow get a multi-million dollar negligence judgement out of the school if something bad actually happens.