Answer :
well, writing report is easy. report is simply the description of what really happened. So, start with how the exhibition started,at what time, who were the chief guests(if any), how was the decoration. what was the main focus of the models displayed (there are topics in science on which the models can be made, so u can describe the main topics that were taken up), how did it all went, what was the reaction of public, etc etc. It will be better to start by the description of the school's decorations and preparations