Can you help me please.

I purchased an app earlier which allowed me access to download the catalogue of movies, I asked for confirmation and was told that the developer had purchased a license and it was legal for me to use here in the UK. As my intention was to show a movie in the school I need to ensure it is all above board and squeaky clean. I was sent a link to the IMDb site as proof they had the license to allow me to do so but I am none the wiser. Can anyone shed any light as to whether I am legally able to do so.