I don't think they can withdraw a place based on behaviour during a trial morning but if you are very concerned, have a look at their exclusions policy.

Hi I need your advice with regards to the place we have been offered in a school. My dd has been accepted by a private school for a reception place next September. It is a selective school where dd had to do one to one assessment and she passed. We paid the deposit and signed the form to accept the place and we are now invited for a trial morning this term with all other new reception children.

My paranoia is that if my dd misbehaves or shows concerning behaviour during the trial morning, could the school withdraw the place? She can be unpredictable occasionally, very excitable and can play up beyond the average level).
My gut feeling is she will be all right, but I just feel very anxious about what consequences it might have.

Thank you for your advice...