Science Class

But astrology is fun, religion is work

And astrology doesn't give you the ultimatum Believe me or I will hurt you. Which nearly every relegion does. (Our religion has the true savior and his principles! Believe me or rot in hell/eternal damnation!@@@)

Also most kids are athiests(Or believe they are) and would probably either

A) Agree
B) Laugh about it.

Teacher really can't do more than tell you to stop considering (at least in my rulebook) doesn't really ostrisize the religion.

you can't prove them to be a pseudoscience though, because they don't make any hard and fast predicitons. jesus is coming back some time, you won't see anything in the bible saying exactly when though.
astrology on the other hand, makes the astrologer give predictions as to when and what exactly will happen, so it's easy to prove as a steaming pile that only a rube would believe in.