In the passive voice, the subject is acted upon, rather than acting, and often the agent, the person or thing doing the action, is left unstated or implied.
In your two examples
The subjects are "Photographers" and "Our teacher" and the objects are "Brad Pitt" and "lots of homework"
If I were told simply to convert these to the passive voice I would write:
In both cases there is no stated agent. We are not told who took the photos, or gave the homework. Note also how the verb has become a form of "to be", and the old subject is now the object of was/were.
One could also write:
Note how the agents is again stated, but is something of an afterthought? The verb remains "to be". The old objects are subjects, but passive subjects.
Now let's look at your suggested versions:
- As Brad Pitt left the restaurant, he was photographed by photographers.
This is passive. It increases the emphasis on the time ("As he left the restaurant") compared to my version, and it uses a pronoun as the formal subject, which isn't strictly the assignment. But I don't quite see why this was marked wrong.
- Brad Pitt had left the restaurant when he was photographed by photographers.
This is a different form, placing even more emphasis on the time, and again using a pronoun as the subject. It is not grammatically incorrect, but it has a subtly different meaning from the active version. Instead of telling the reader what happened ("Brad Pitt was photographed"), it assumes that and tells the reader when it happened.
- Lots of homework this week has been given us by our teacher.
This is awkward, and possibly grammatically incorrect. "This week" is improperly placed, so that the subject becomes "lots of homework this week" which is either wrong or has a very different meaning from the intended one.
- Lots of homework have been given this week by our teacher.
This does not agree in number. "Lots of homework" is singular, and so the verb form should be "has" not "have". It also omits the old object "us" and so does not say who received the homework.
- We are given lots of homework this week by our teacher.
This changes the sentence from past to present, and is also an awkward formation.
- This week we are given lots of homeworks by our teacher.
This seems to suggest that "this week" is different from other weeks, and emphasizes tht. It also is in the present, not the past, as the original was.
As the [grammerly blog] puts it:
While tense is all about time references, voice describes whether the grammatical subject of a clause performs or receives the action of the verb. Here’s the formula for the active voice: [subject]+[verb (performed by the subject)]+[optional object]
Chester kicked the ball.
In a passive voice construction, the grammatical subject of the clause receives the action of the verb. So, the ball from the above sentence, which is receiving the action, becomes the subject. The formula: [subject]+[some form of the verb to be]+[past participle of a transitive verb]+[optional prepositional phrase]
The ball was kicked by Chester.
That last little bit—“by Chester”—is a prepositional phrase that tells you who the performer of the action is. But even though Chester is the one doing the kicking, he’s no longer the grammatical subject. A passive voice construction can even drop him from the sentence entirely:
The ball was kicked.