Label the Sentence: 9-25

Because there is an escaped lion in the school 

halls, we are going into lockdown.

Label this as a complex sentence.

Dependent Clause,  Independent Clause

A complex sentence combines and independent clause with one or more dependent clauses.
When the dependent clause comes first, there is a comma between it and the independent clause.

"Independent clause" is another way of saying that it can work as a complete sentence. 
A dependent clause has a subject and a verb, but it cannot stand alone as a complete sentence. 
In the sentence you copied 

"There is an escaped lion in the school halls" could  be used as a complete sentence.  However, once the word "Because" is added to the beginning, it is no longer a complete sentence.  It has become dependent and cannot stand on its own. 
It needs to be linked to an independent clause.