Dear Serina Knight
But it depends partly on what kind of climate you live in and what kind of things you do. The bare minimum you should drink is a litre a day.
But if you were, for example, walking all day across a very hot, dry environment, you might need to drink up to four litres a day. Adults might need to drink more. You see, it depends on what you are doing and what conditions you are living in.
Usually your body will tell you if you need to drink more because you'll start to feel thirsty.
Some people, girls especially, learn to ignore thirst during the day because they don't like having to go to the toilet at school. That often causes problems with things like headaches and not being able to concentrate. Many of these problems can be solved quite easily just by drinking water--which is very simple, safe and cheap.
You definitely should have something to drink before you go to school and ideally you should drink at school as well and either develop a very large bladder or brave the toilets.
And remember...the first thing to do if you get a headache is just have a drink of water. That fixes most headaches straight away.
There is one exception to thirst being an indicator that you need to drink. If you are having heavy exercise like running or playing sport, by the time you actually feel thirsty you are probably already starting to get dehydrated and your body won't recover from it while you continue the activity. This can be very bad for you.
So always have reasonable-sized drink before heavy exercise...and make a plan for being able to keep drinking while you continue, especially on a hot day.