The most important thing to do is listen and talk to your child. The next thing is to report it. Speak to Mrs Stretton or Mr Wheeler in school.
Watch the following video for some good advice
Follow the link below
https://www.nspcc.org.uk/preventing-abuse/child-abuse-and-neglect/bullying-and-cyberbullying/
The NSPCC has lots of tips and support for parents on what to do if they think their child is a victim of cyberbullying