DS starts school next year and I'm trying to get prepared. What are the preferred drink bottles? I currently use Ecococoon 600ml insulated stainless steel bottles for him which work great for preschool and home but I think might be too heavy for him in his bag. The school suggested one drink bottle for the desk and one for the bag so 2 of those are going to be pretty heavy. So I'm looking at plastic ones but I'm worried about them sweating on the desk if I put cold water or ice cubes in them. What do your school kids use?