It just has to be looked after just like you would at home. Kept an eye on so no kids do nasty things to it, and it has to go somewhere during the holidays to be fed and have water change and tank cleaned etc
someone have to look after it and fed it even during the holidays the same way as if it was at home and look out for kids can do nasty things to it onder weekends you can use special fod for weekends
If they are true goldfish (as opposed to the tropical varieties), they should survive well with a few plants and an air stone (though with goldfish the air stone is not really essential). Keep a lid on the tank & avoid overfeeding at all costs...I imagine this would be difficult depending on the age of the kids. Perhaps place it high so they have to look up at it and the person who feeds it needs to stand on a chair or something. While it is important for kids to learn about living things and fish tanks can be really relaxing, I always sort of pity the fish that get kept in classrooms.
...Oh I also meant to say that for weekend feeding and even for up to a week, you can get slow-release food tablets, but for anything longer than that, they need to go home with someone.....god help them
and please ensure that the tank/bowl is at no time of the day completely in the sun. Even on a cold day a tank can heat up to an alarming temperature and effectively overheat your fish, especially behind window glass.
are you planning on teaching them about death? because these poor little guys will be dead in a week - get something a bit hardier like a gerbil or a rat (e.g. something that can fight back a bit)