Question: Is there any difference between considered to be and considered as?

For example:

* Adam is considered as a good teacher.
* Adam is considered to be a good teacher.

Answer: 
"is considered to be" is [significantly more common](https://books.google.com/ngrams/graph?content=is+considered+as%2Cis+considered+to+be&case_insensitive=on&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cis%20considered%20as%3B%2Cc0%3B.t1%3B%2Cis%20considered%20to%20be%3B%2Cc0) 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](http://dictionary.reference.com/browse/left-handed+compliment). 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).