I think you have a very insightful and honest friend and you should thank her for caring enough about you to say what she thought, she risked hurting your feelings, but you should listen because she is right.
It sounds to me like you may have already over stepped the typical understood boundaries for school birthday celebrations, at least for every school my kids were in. It would bother me to be in the position of not being able to provide preschool for my child if I wished to decline your invitation (if my child was afraid of clowns, for instance.) I might also feel funny and obligated to pay for the time my child was there, if that was not a day that they would ordinarily attend. Even if my child was not very close to your child, if I recieved an ivitation (registry or no registry) I would feel obligated to bring a gift, and that kind of puts people in a situation where they have to buy something for your daughter to go to school that day. I think a registry is tacky unless you are a bride or a new Mom.
If you want to provide a fun time for kids at school, then do that, but when you have a full birthday party during common school time, you are expecting a lot of people who depend on preschool for utility. I am sure you did not set out to do that, but some of the parents in the school may be looking at you that way.
As your daughter has more parties you will see, some parents will call you and ask what she wants, and some will not, and you are probably better off to leave it at that. Birthdays are the perfect time to teach children to be gracious about every gift they recieve, regardless of whether they got what they really wanted or not. I would suggest that you go ahead and hire a venue next time, and leave out the registry on any invitation you send out for this party, and the parties to come, and that you learn a little life lesson from the experience! Maybe most of all, listen to your friend, she seems to have your best interests at heart!
M.