Difference between "considered to be" and "considered as"?

"is considered to be" is significantly more common and if you look at other uses of "is considered as" you notice a key difference between the two sentences:

... who is considered as a debtor...

... the thing whose representation is considered as a part of the sphere...

These uses are either telling the reader that you should (a) consider two things as equals or (b) use a particular context in order to consider something.

"is considered to be" is telling the reader how others consider a thing. In your example, this is much more likely to be the correct choice.

Adam is considered to be a good teacher. — Adam is thought to be a good teacher.

Adam is considered as a good teacher. — We have treated Adam as if he is a good teacher.

The difference is subtle and not easy to explain.

In addition to Mr. Hen's correct statement:

Considered as can have another meaning: to think about in terms of.

"Adam is considered as a good teacher" can mean people decided to sit around and think about him as a good teacher. (This is subtly different from Mr. Hen's treated as if he is a good teacher.)

Context, of course, makes this unlikely.

In AmE, the more common constructions would be considered to be, or even considered (a to be deletion).

Adam is considered a good teacher.

Also worth noting: Considered or considered to be, may be left-handed compliments. It may carry the implication that, given the lousy performance of all of the other teachers, Adam is considered a good one (despite his otherwise glaring incompetence).

There is no such thing as "considered as"

Adam is considered a good teacher.

http://www.thefreedictionary.com/as

As is sometimes used superfluously to introduce the complements of verbs like consider, deem, and account, as in They considered it as one of the landmark decisions of the civil rights movement. The measure was deemed as unnecessary. This usage may have arisen by analogy to regard and esteem, with which as is standardly used in this way: We regarded her as the best writer among us. But the use of as with verbs like consider is not sufficiently well established to be acceptable in writing.