Pica and chewing on things

Hi, my 3 year old son has always put things in his mouth. We've got chewies, always watch him and proofed the house and garden as much as we can..However he started school a few weeks ago part time and despite it going as well as it can he has been putting sand in his mouth. He's still in nappies so once he's pooed it's quite obvious that he's been doing it obviously. We had meetings with the school before he started and mentioned this. They've proofed the class of small things but not the sand. I understand sand is a big thing in nursery classes which obviously isn't great for us. We've always avoided sand really, he's put soil in his mouth in the past but we basically put fake grass in the garden so he couldn't do it. We are going to speak to the school but please does anyone know of any way to stop him doing this? Its so stressful. We can mostly stop him at home but now this has happened in school it's upset me. I just feel like noone can help us tbh