Been like that pretty much constantly, Yoshiko is the only one whose panties can be shown while still wearing them.
There's one exception of some newsperson on TV in the storm chapter, that one surprisingly showed worn panties without being Yoshiko.
You do occassionally see others panties when not worn though, like when crazymom went through Sayaka's home searching for less childish ones,
or anytime crazymom or miss discipline snatched ones from eachother and/or others.
Edit: Forgot to mention I loved that entrance at the end of this chapter. =D