It is true that as may start a clause but as can be used for other purposes too:

- an equivalent to 'since' /seeing that
He wore a snorkel, as his abilities to hold his breath are not great.

-'as' can also be used for the word 'like' but be careful!; as is usually before a noun followed by a verb whereas 'like' compares just nouns. In some cases as in the one you brought up 'as' is similar to 'like' but with a bit of a different meaning. 'As' an adolescent means she is an adolescent.

Example:
I worked like a slave I worked very hard but am free)
I worked as a slave (I was a slave)

"as" is also used as a synonym of 'in the capacity of'. So, following would be correct:

As teachers, we have responsibility towards students.