Yes, you're correct; the most recent (and most powerful) is the Pi3. This would be the most suitable Pi to get for someone as a gift, in my opinion.
There are a number of resellers, depending on which country you are in. I'm going to take a guess (based solely on your use of the term 'elementary school') that you are in the USA. Adafruit and Element14 are, I believe, the main resellers of the Raspberry Pi over there.
You can buy the Pi on its own, or as part of a 'Starter Kit', depending on what accessories you expect the recipient to already have access to.
The following kit from Adafruit would give you everything needed to get going:
Hope this helps