UHT fresh milk or formula?

Hi mummies, I’m in a dilemma here. My son will be going to playgroup in a few months’ time and he’s still on full BM now. I’m a SAHM so this will be his first time going to school. I understand that the school only accepts packet milk or formula. I prefer to give my son UHT fresh but each packet is at least 200ml, and my son can only drink 100-120ml each time. Is there any UHT 100% fresh milk that comes in small size (less than 200ml)? Or is it possible that he drinks half a packet for breakfast and the other half before nap? Seems like UHT milk needs to be refrigerated once opened, and I don’t want to trouble the centre to heat up the milk just for him. Appreciate any advice. Thank you! #pleasehelp #advicepls #worryingmom #firstbaby