Rhubarbs post is spot on.
Our son spent two years at a very good, oversubscribed Catholic Faith school. Although it was very academic oriented, he only learnt religious songs, and his talk about God and Jesus was slightly unnerving at times, even for us who are believers (I am a Christian, dh is Catholic).
The other thing is that from my experience, the other mums were either middleclass sahms with 3-4 kids, some planning more, or middleclass workaholic mums with nannies for their 3+ children. The level of involvment with the school was very high, with parents being requested to help staff the library, come in and read, come on outings, bake bake bake for cake stalls, take part in religious festivities in school, but most importantly be seen on the sunday, take voluntary roles in the Church AND CHAT about the service...
MOST IMPORTANT:
As we came from outside the parish, and lived a bit away from the school, going for sunday worship was really not part of our priority. My son therefore did not already know the other children in his class, and they all knew eachother from Church and sunday school. Very few mums were keen on arranging playdates with my son. I had to work really hard, and although they were friendly, my son was not really accepted.
In the end, it was my au pair befriending the nannies which was the ticket to playdates.
Think carefully.