Interpretations

I'm annoyed, though not surprised, at all the posts interpreting this film as being about sexual awakening and/or loss of virginity. While a decent case can be made for it in Irma's case (except we're explicitly told she remains a virgin after she re-appears), it doesn't make much sense in Miss MrCraw's case, nor is it consistent with Mrs. Appleyard's death, as well as leaving lots of details unexplained. I think this is mainly due to an unfortunate tendency to see films about teenagers automatically as coming-of-age films (implying teenagers cannot be characters in their own right, but merely act as bridges between childhood and maturity) and a tendency to see coming-of-age as primarily sexual (as if sexual experience was all there was to maturity - obviously it's natural for coming-of-age stories to have a sexual dimension, but that doesn't mean it has to be at the exclusion of all else). The girls becoming grown women was something people would both expect and wish for, so a disastrous disappearance would be a rather strange metaphor for it. It warrants another look at several key elements:

Partial nudity: Nudity isn't automatically sexual, esp. partial nudity. Here, it starts with the removal of gloves - this has to be significant, as we're shown both the authorization and the actual removal. The literal explanation is simple: gloves would be too constricting on a warm day - Mrs. Appleyard clearly did not intend it to make the girls look more attractive. Then, when climbing on the rock, they remove their shoes and stockings - going barefoot is not justified by practicality, given the presence of snakes and that walking barefoot on rough terrain is not very comfortable if you're not used to it. Now, barefootedness is fetishized by some, but it can have other connotations (in some countries, abject poverty), but in any case it is not customary in Western cultures to hike barefoot and certainly not in the specific context - it's a very blatant violation of social norms, as is the teacher's partial nudity (but not exposing breasts or genitalia) Edith later describes. Finally, there is Irma's missing corset, about which we're told after we're told she's "quite intact" (virgin). A corset is an extremely constricting item, particularly for anyone engaged in "tomboy foolishness" like hiking - again, it ties into a rejection of constraints imposed on the girls - the removal of items of clothing follows a pattern of removing of restrictions, all the more so because it's a school uniform. Edith, who does not go beyond removing her gloves, does not disappear.

The journey: They leave the school - obviously a place of conformity - for the picnic area, still a "tame" place but less "civilised" than the school. The girls then leave the area - with permission, but it's clear Mrs. Appleyard would not have approved. Then they completely break the ruled by hiking onto the rock. Mrs. Appleyard had forbidden it, not for being "indecent" or an action reserved to adults, but as "tomboy foolishness." That is, it would be a violation of gender norms (and thus, not an injunction applying only to the young).

Irma's reappearance: Instead of being welcomed back, albeit in a new role, as one would expect if her disappearance were simply a coming-of-age metaphor, she's violently rejected by the other girls, and Mrs. Appleyard knows from the start she will not re-join the school.

The two guys:They too visit the rock, although they do not disappear. The scenes with them drinking, commenting on the girls' appearance, etc, given the different social classes, violate conventional norms (accounting for the awkwardness). They're the only ones who have any kind of contact with the girls after their disappearance (Irma). Some viewers find sexual tension between the two guys, which would obviously be another violation of social norms.

Appleyard's death: Appleyard's existence is dedicated to "respectable" ends - turning girls into socially acceptable young women, motivated by profit. The disappearances ruin this (which makes no sense if the disappearance is a coming-of-age metaphor). She "loses it" and herself throws caution to the wind, engaging in drinking and otherwise not abiding by norms of expected behaviour. She attempts to climb Hanging Rock only to fall to her death - the place where the girls had disappeared destroys her, an enforcer of conventional behaviour, financially, socially and finally physically.

My interpretation is that the girls' disappearance, and Hanging Rock, are symbolically about rejecting conventional behaviour and values - they "disappear" having either become outcasts or left the world they were formerly a part of. There is no reason why this should apply only to teenagers, so Miss McCraw's disappearance is not strange in this context. Irma's rejection makes sense if she's become a social outcast. The watches stopping implied nobody is being timed anymore, which fits. In this view, Sarah commits suicide because she has no other way of escaping the constraints imposed by others (she's prevented from going to the picnic) and her situation is of greater oppression than that of most of her schoolmates (with the threat of being sent to an orphanage and her being a probable lesbian). Finally, the in-universe mystery itself fits - a metaphor for most people not being able to fathom why some individuals would reject social norms and leave that particular society.