I was surprised when I looked on IMDb to see how many versions there are listed of this story. There are TONS--and in many different languages--all beginning in 1915. However, the story predates this, as it was a VERY successful London play in the 1890s--with over 1400 performances! Because of this, there's a very good change you've seen one of these versions or films that seem to have been influenced by "Charley's Aunt" (such as "Tootsie" or "Some Like it Hot").
Moore Marriott and Graham Moffatt were most often seen in films supporting Will Hay. However, after quite a few successful films together, the pair began working with Arthur Askey--another very successful film comedian. Moffat and Richard Murdock are students at Oxford--though how these boobs got into the school is beyond me. The three are constantly in trouble and sooner or later, they're bound to be 'sent down' (that's British for 'expelled'). So, when they finally have pushed too far, the dean sends a letter to their aunt to report their indiscretions. However, the letter is intercepted by Askey and he is persuaded to impersonate the aunt--and try to charm the school officials out of the expulsion. In a twist from the original story, it seems that Askey had learned how to play a woman (badly) by playing the lead in the school production of "Charley's Aunt". In other words, it's like a play within a play. I could tell you more about what happens next, but it's best you just see it for yourself.
This is a pretty typical Askey film. Pleasant and silly, but certainly not brilliant or a must-see movie. It's funny, but my wife had no idea what I was watching but when she came in the room she saw Askey in drag and said "who's that guy". And, if it's THAT obvious to the viewer, perhaps they didn't do the best job in the film.