Why do they have to meet at a school?

In reference to the disagreements about religious groups meeting in schools, I have to wonder why the club members don't meet in their churches?

• Bible club lawsuit roils school

Don't most people who profess a reglious belief, and want to get together in groups, often belong to organized groups who usually have a place to meet?  It's not like many other extra-curricular activities are organized to the point of collecting enough money to own real estate, plus American schools have expanded to the point that everyone expects the buildings to be public meeting places instead of places of education.

If the point of the meeting is prayer, fellowship and discussion, why does it have to take place in a school?

Or maybe those reasons are not the point of having the meetings.