Enough food and water so they don't run out, it will increase as they continue to grow... One day of watching them and you should have a good idea on how much they are eating and drinking, extra food and water never hurts so be generous before you leave for school...
I currently have 18 chicks in the brooder, they are about a week and a half old, they are drinking about a quart of water a day and eating about a 16oz Solo cup of food a day, maybe more... Water I can estimate a lot easier then food as I top off their food bowl whenever it gets low never really pay attention to how much is left or added, water is filled fresh every day so I have a better idea on that consumption...