[Sashimister]

Originally Posted by Xajron

Ok, so I should say 学生で、先生もです? Got it.

Nope. You cannot say もです.　　Please read each letter in my sentences carefully.

You can choose between でもある and でもあります.

So ありis kind of like when you use the stem form of verbs (instead of the te-form) to link sentences, but you use ありbecause だ is already in the te-form?

You seem a bit confused here.

I stated last time that あり was the continuative form of ある. The term "continuative form" is the official name for the "te-form". (Believe it or not, the term "te-form" is NEVER used in Japan, which is why I don't like using it in my explanations.) So, あり is already in the "te-form" even though you see no て in it, and so is で.

This is the reason that you should construct these sentences as:
(Again, read each and every letter carefully or you will learn nothing in this post. If I were you, I would read these aloud.)
であり　+　であり　+　でもある or
で　+　で　+　でもある

でもある can be replaced by でもあります.