Do teachers have legal recourse when parents are aggressive toward them?

Certainly, the question you've asked is a sensitive one, no question about it. Harassment of teachers is under-reported, but if recent news stories are any indication, they are increasing. If you're asking about the United States, there are no specific federal laws that address parental harassment of teachers at this point in time.
Some states have taken action to protect teachers, however. In 2017, Utah began debating H.B. 62 regarding educator rights amendments. Meanwhile, in 2012, North Carolina premiered the first cyber-bullying law in the United States: the law protects teachers from cyber-bullying by students and others.
From this page, you can research bullying and cyber-bullying laws in each US state. Note, however, that the laws are mostly directed towards protecting students. Again, there are no federal laws that specifically address bullying. In the UK, the problems are similar. There are currently no laws that protect educators from cyber-bullying in the UK. Meanwhile, the NASUWT, the largest teachers’ union in the UK, reports that almost 50% of parents are taking complaints about their children's teachers to social media.
The latest study that has attempted to document teacher abuse is one by the APA, which states that 80% of teachers have been victimized at least once during the school year. The APA report also points to a 2013 Bureau of Justice report, which states that 6% of K-12 teachers have been attacked in schools. So, the problem is definitely a serious one. The APA calls violence against teachers a "silent national crisis."
Again, teachers have few legal resources to deal with aggressive parents. The problem is further exacerbated when principals are cowed into keeping silent about abuses. Increasingly, teachers are reporting that so-called zero tolerance policies do little to protect them against parent perpetrators. Some schools, however, allow the educator to call the police in the face of physical threats.
From what I can see, teachers who feel threatened can either refer the problem to their school administrators, report incidents to the teacher's union, or call on the local police for help.