Toddler started to strip naked going toilet

Hi,

My son 4 year old has recently started to strip naked when going toilet. I have tried to discourage this but it seems to be getting worse. He has just started school And they said he has started doing it there. He has been great when going toilet until recently and never had any problems with this. Does anyone have any tips on how to stop him from doing this?

Best wishes and thank you in advance