Party or Parent pooper?

I have a situation that I need a little insight on.
When you throw your child a birthday party and send the invites to school with the child do you invite all the kids or just certain ones?
Any thoughts from someone who's gone through something like this? Do you feel that invites should be sent privately through mail or email so a child doesn't feel slighted around peers? Or even to avoid situations like mine...leaving an invite in a child's backpack only to hear about it leaving you as a parent clueless? And if a parent approached you with a valid question, not berating you for not inviting their child, but asking a valid question with a genuine concern do you blow up and act as if they are beneath you and your party?

I don't know...I really would love to hear from some moms out there and how they feel about these situations. I know for my daughter when I plan her party I always plan for 25 kids because that's what normally around what shows up. I make a conscious effort to leave no child out. It's up to the parent to do what they wish..but I know I extended the invite. I mean after all this is who it's all about right? Our children??