Our girls are thirsty very often.  They are in 2nd grade and kindergarten and keep a water bottle at their desk.  They have been drinking from waterbottles for a long time so we’re not concerned that they will spill.  The teachers are great about it and since it is right there, they have no distractions or embarrassment about drinking out of the water fountain or getting a bottle out of their book bag.  (Plus I always get a bit antsy about all the germs at the drinking fountain.) They have always loved water.  From the time they were little I had them drink plenty of water and also made them "juice cocktails" 1/2 water and 1/2 juice.  Keep the liquids flowing- if their thirsty or their body is craving salt, they’ll let you know.  :)
Tonia