There are a few options for you:
1) A good resource is the ‘Selective Mutism Resource Manual’ by Wintgens and Johnson
2) Our clinic runs regular 3-hour evening seminars for teachers about how to support students with selective mutism at school – https://selectivemutism.com.au/seminars/
3) Our previous seminars have been professionally video-recorded, and available to purchase on the website here: https://selectivemutism.com.au/product/treatment-schools-dvd/
4) I would also encourage the student/family to engage with a psychologist so that they can receive treatment for the selective mutism. As part of that treatment the psychologist should support you with strategies for the classroom.