In my Biology class, the question, "if alien exist always pop out"!

My common answers is:

1st: up to now there is no ET enrolled in our school, nor some declared in a noontime shows an alien participating in their games.
Why arent they coming out? Maybe there is REALLY NO ALIEN OUT THERE!

2nd: If i told you that there are really aliens that exist among us, who roam our planet, and we are visited by numerous aliens from the past, and (jokingly) I also came from the sky, then what will you do? Will your faith in God differs, will you not panic? will you still be interested to go to school?go to your church, what will you feel?

As I process the discussion:
Saying on the class
The question of existence is just a mental experiment.
Its your conscious knowledge and its acceptance will matter if the truth came out.
And the continue valuing life and our species matters to these aliens too.
How can they treat us an equal lifeforms if we dont love our own.
If they are really the gods of the ancient, will you are ready to listen?
Will you protect the Earth now that aliens do not exist, or you will protect the earth knowing they do?

My students have no follow up questions, the bell rings.

I really like to entertain those kind of questions, somehow they are quite satisfied.