https://play.google.com/store/apps/details?id=com.spyparent.bullyblock&hl=en

bully block will allow you to capture the inappropriate texts or if there are calls etc.

My neighbor friend had a situation that escalated but started very similarly to yours and the police had an issue because much of the communication leading up had not been recorded because of snap chat etc.

then I'd immediately follow up with the perpetrators parents as others had suggested.

If you are really concerned I'd also notify the school so the teachers can keep an eye out for any escalation of physical or verbal abuse. they take that stuff very seriously if you have text proof.