You can read about the whole situation on this press release from the ACLU.
'McMillen said that she plans to attend the "private" prom, but has also long planned to attend the Mississippi Safe Schools Coalition's Second Chance Prom, to be held Saturday, May 8 in Tupelo. That event, sponsored by Green Day, Tonic.com, Iron Chef Cat Cora, and Lance Bass, among others, will be open to all LGBT students in the state, as well as straight students who are LGBT-supportive.'