Please help me on this and please follow the directions ​

Just explain what you would if you was locked in the classroom. Like call your parents or have fun in there. Anything you would do.

Just write exactly what you would do. There are no boundaries for this question since it asks only for YOUR opinions.
For example:-
I would freak out for a little while. I would roam around classrooms and the playground. I would talk and sing as I like. I would sit outside and have some snacks. And after having some fun I would call my parents and break the news to them and wait for them to take me home.

Yes. He saw them has a subject (he), a verb (saw), and an object (them), so it is totally a sentence.