Are you concerned about the timing of your DD being told or the fact that she has been excluded?
Are you worried about the costs of proms and of children feeling they must have partners and generally pressue over proms?
Your posts don't really make this clear and both things seem to be mentioned, but a litttle muddled (at least for me on my second Pimms ;) )
The school were definitely wrong to tell her just before her exam. That os not on, and something they should apologise to your DD for.
As for excluding her - not sure. Depends on the behaviours that have led up to this. If she is merely standing up to protect others from bullies then this needs investigating further obviously. But then there seems to be more to this than just that TBH.
As for the money - parents should simply not go along with it and it'd stop. OK - buy a dress. But TBH I go to several dinner dances and balls and my dresses are always much less than what you are quoting. You can get sick floor length dresses in John Lewis for £150 and less, and a LOT less! They do not need limos etc. There is no need to spend this level of money and it is the parents who are daft if they chose to do so. And TBH, if a child was already on report and this was a threat, then as aparent I would definitely not be getting the dress til much close to the date (I know this is not applicable to you).
Yous ay you have made alternative plans for the night of the prom. Is this because you feel your daughter has been excluded unfairly?
he FB is a red herring and irrelevant. There are petitions on there all the time and they mean nothing. Some people will join the opening of a stamp!