Roberta-
This is not unusual for a child going through divorce.
At this age they can think it is their fault or think they are no longer
loved.
Anger , being a secondary emotion, has a root in fear . Be patient, be firm,
and insist that she either complies with kind behavior( pair her with a
sympathetic friend)or she sits alone.
Take the initiative and call Mom, she could use the information and work with
you.
Accountability is a big PLUS.
Patti