For one thing, what she's doing is both illegal and totally deranged. Besides which, even if you were having a relationship with her ex-husband, it would be none of her business! If it was a man doing this to their ex-wife, I wonder if those saying don't inform the school would still be advocating the same.
More importantly, informing an employer af the poor conduct of an employee is not harassment of the employee. If anything, it's doing the employer a massive favour. You would not be telling the employer lies, you would not be passing on malicious information. You would be informing them of facts and actual events that have taken place, as well as the fact that she's being investigated by the police. What the employer decides to do with that information is then up to them.
In many professions and in many high level jobs, your conduct outside your professional life and your work, wherever it may may be, can be monitored, scrutinised and poor conduct heavily punished. This is often the case for doctors, teachers, lawyers etc. for brining the profession into disrepute. Any employee of any company may also be disciplined, including being sacked, for bringing the company into disrepute, too. Poor conduct of employees in their private lives can cause significant adverse publicity for both public and private institutions which can be highly damaging if not dealt with appropriately.
I would certainly want to be aware if an employee is behaving in this way. A reasonable employer will check the accuracy of any information they receive and act accordingly with appropriate legal advice, especially a school. They won't just go in all guns blazing.
This is a potentially unstable person in contact with children. If your allegations are true, I wouldn't her anywhere near mine and I would hope that the school would be informed by someone, anyone, of what's going on. For that reason alone, I would ask that you do contact the school and inform them please.