A loft is a very handy kind of space, with practically no restrictions attached. Typically, these spaces are free of furniture, so you can bring your own and set it out as you please. As a rule, lofts have high ceilings which measure at least 4 metres. Many of these venues are located in industrial zones, on the premises of former factories and warehouses. This should mean that you are free to play loud music and dance.
A school leaving party is an official event. The venue should be clean and have good lighting for photography. Don't forget to check the acoustics, as there may well be speeches involved.