ABANDONED. YA series read in early 00's, group of friends that communicated through secret messages sent between their bedrooms through a string

[Letícia]

I LOVED this series when I was a teenager, but all I remember from it was:


There was a groupa of female friends, and each book delved deeper in the story of one of the girls. They had a secret, something to do with aliens or monsters, and their 'boss' was a giant talking frog that was a bit of an asshole, and he gave them missions to complete. they lived near each other and used secret messages to communicate, they sent the messages through string attatched to their bedroom windows, and the messages were sent in a secret code (the meaning of each symbol was in the back of the book so the reader had to translate each message).
I remember my favourite was a girl who lived with her younger brother and mom, her father had passed away/left.
There were all still in school.

Please help!!

[Lobstergirl, au gratin]

Leticia, are you still looking for this or did you find it?

[Lobstergirl, au gratin]

No response; moving to Abandoned.