While they don’t specify the new rules, there are a few clues in the arguments made to defend it:

• Teachers shouldn’t be measuring clothing. This implies the new rules are not based on any kind of measurement
• That leaves binary states of whether X body part is covered
• The article states that exposing the midriff is among the newly-allowed items
• They also argue for leaving it to the parents to decide what’s appropriate. This actually seems to imply the new rule is “anything goes [so far as school enforcement is concerned]”