http://en.wikipedia.org/wiki/DruidIt says in the wikipedia article that there is next to nothing known about Druids except that they were part of the priestly class in different parts of Europe. The article also implies an association with nature. While Wikipedia might not be the most reliable source about specific facts, getting general background info about something can sometimes be okay.
Anways, Druid vs Necromancer sounds like "Light versus Darkness" AND "Life versus Death". It seems most likely to me that the Druid will either
1.) have both Nature and Holy training but everything out of school costing triple, or
2.) the Druid might have the choice to be schooled in either Nature OR Holy. But since we already have the Priest and Priestess having another Holy only mage seems like a bad idea at the moment...
3.) the Druid will be a nature only mage. She will have somewhat of a religious background but not be of the holy school, since she finds spirituality out in nature instead of inside a man-made place of worship like a church or temple as is normally expected of the priest class in RPGs.
And also, why would the necromancer have water when he's fighting against a Plant-user? If anything he should have Earth, since that's where the bodies are buried, and that way he can uproot plants as well as bodies. I think it more likely that the expansion will have Earth support for both mages without them actually being trained in it. The Druid is trying to put things in the ground, while the Necromancer is trying to take things out of the ground. Also, maybe some water support, but again, not having the Druid trained in water makes more sense, since there are plants that exist which need very little water to survive. While they still need water, so does all life, so I don't see why she should have particular training in water. I can see her getting training in Holy because Holy has a light theme going and plants need light for photosynthesis, but Water? Definitely not, especially since I don't see that doing very much against a Necromancer using Dark and possibly Earth as well.
Druid used splash!
Necromancer's graves are muddy! Undead creature spells are now easier to cast!
Of course, maybe instead of just being a plant version of the beast master, she should have training in ground after all to set her apart rather than just receiving ground support. But if she does receive ground training, then the Necromancer will have to receive ground training also, because it just wouldn't make sense otherwise.