Shark is correct. A 2/1 creature is considered a level 3 creature for the purposes of spell effects and creature abilities. A level 2 nature / level 1 water spell is a level 3 spell and is considered a water spell as well as a nature spell. However, for purposes of spell book creation, the Druid only pays 3 points to add it to her book.
Schools of magic are a different category of minor and major. No spell or game effect currently cares whether your spell belongs to a major or minor school of magic--it's merely a lore designation and (correct me if I'm wrong) a measure of how robust the scope of that school is meant to be. I believe that the elemental schools are the only ones designated as "minor" at this juncture.
