Good question - let me attempt a good and hopefully helpful AND correct answer!
Simply stated this is a school that has not met it's NCLB Annual Yearly Progress (AYP) Goals in one or more subgroups for three years in succession or longer. Note that these need not be the same subgroups missed in all three years.
As such PI 3+ is a category, lumping PI 3 with PI 4 and 5.
NCLB has no PI 6 (apparently the counters were limited to the fingers on one hand) so PI schools accumulate pluses after year five.
PI Advancement Criteria: Once in PI, a school or school district that fails to make AYP will advance further in PI status.
PI Exit Criteria: A PI school or school district that makes AYP for one year will maintain the same PI status for one additional year and be required to continue implementing the applicable NCLB requirements. In order to exit PI, a school or LEA must make AYP for two consecutive years.
Onward! - smf