The movie is not a classic by any stretch of the imagination, but it does have its good points, some of which I found interesting/ enjoyable. To get the negatives out of the way first, there isn't much of a plot in The Trouble With Angels: it's more a series of vignettes of events which occur involving the girls and the nuns- mostly the Mother Superior. Also, there isn't much character development for anyone except the three main characters. The rest of the school girls are more or less indistinguishable, and the other nuns hover at the periphery, never getting much screen time or development. This makes the scene when one of the sisters dies during their senior year much less impactful than it could have been, because she is never a character that feels real to you.
It's clear that in this film Hayley Mills was attempting to step away from her Disney persona, playing the rebellious teen Mary, smoking and talking cynically about her uncle's affairs. And at nineteen, she's still young enough to pull off playing an adolescent between the ages of 14 and 17. Some of the other girls, however, are very obviously not teenagers. For most of them, this isn't really important- they're only background dressing- but June Harding, who plays Mary's best friend/partner in crime, was 28 at the time of filming. While she skilfully portrays the awkward and gawky Rachael, at no time did I believe that she was 14 years old.