I have to say I'm not sure what to make of this film. It does have its flaws, yet I already feel an affection for it, having seen it only twice.
Flaws first - as expected with Dario, the acting isn't exactly Oscar quality, but hey, you don't watch an Argento film for the method acting. Jennifer Connolly has the right vulnerable look, but at times you want her and the other actors to put more oomph into it all. The great Donald Pleasance however is always watchable.
There is more plot than in most Argento films, but the setting of a girl's school could be spookier than we see here (see Argento's own "Suspiria" for proof). Sets are faceless, and seem like a series of unrelated buildings - we see them once for a set piece and never again. And you could at least give some victims a bit of sense - if you know there is a killer about, don't go out on your own! And why keep a girls' school open when a teenager-slaying psycho is on the loose in the area?
However - there are some neat set-pieces - the use of insects is clever and never feels twee or dumb, which it easily could. The sleepwalking effect is clever (despite some dodgy 80s rock video corridors!) The set-up for the climax lasts nearly 20 minutes, but when it does come, the pay-off is good. Sadly, at the very end it goes a bit panto.
I still enjoyed this film more than "Inferno" - "Phenomena" at least has more structure. It may be flawed, but it is still better than most horror from the 1980s, which is why I give it four stars. Just try to ignore the dire Iron Maiden song that pops up from time to time.