Remember, you can't talk to ALL characters that you see, only specific ones.
The majority of ones you can talk to get added to your 'Contacts' menu on your back button. Some characters appear in different places at different times, but I'll be listing where I found them.
Frankie Stein - Courtyard (Morning) - Opening Cutscene
Sirena Von Boo - Courtyard (Morning) - In front of school near opening cutscene
Bonita Femur - Courtyard (Morning) - In front of school near opening cutscene
Neighthan Rot - Courtyard (Morning) - In front of school near opening cutscene
Twyla - Level 2 North Hall (Morning) - Group of 3 near Attic entrance
Howleen Wolf - Level 2 North Hall (Morning) - Group of 3 near Attic entrance
Gigi Grant - Level 2 North Hall (Morning) - Group of 3 near Attic entrance
Mr Rotter (doesn't get added to contacts) - Level 2 North Hall (Morning) - in Clawculus corridor
Spectra Vondergeist - Ectonomics classroom (level 2) (Morning)
Abbey Bominable - Ectonomics classroom (level 2) (Morning)
Catrine DeMew - Ectonomics classroom (level 2) (Morning)
Toralei Stripe - Ectonomics classroom (level 2) (Morning)
Lagoona Blue - Ectonomics classroom (level 2) (Morning)
Elissabat - Level 2 South Hall (Morning) - Near the south west part of the corridor, according to the map
Operetta - Library (level 2) (Morning)
Robecca Steam - Library (level 1) (Morning)
Draculaura - Library (level 1) (Morning)
Clawdeen Wolf - Library (level 1) (Morning)
Purrsephone - Foyer (Morning)
Meowlody - Foyer (Morning)
Gil Webber - Foyer (Morning)
Manny Taur - Foyer (Morning)
Bloodgood - Foyer (Morning)
Invisi Billy - Auditorium (level 1) (Morning)
Ghoulia Yelps - Auditorium (level 1) (Morning)
Mr Hackington (doesn't get added to contacts) - Mad Science classroom (level 1) (Morning)
Gilda Goldstag - Quadrangle (level 1) (Morning)
Jane Boolittle - Quadrangle (level 1) (Morning)
Venus McFlytrap - Quadrangle (level 1) (Morning)
Iris Clops - Quadrangle (level 1) (Morning)
Catty Noir - Swimming Pool (level 1) (Morning)
Clawd Wolf - Swimming Pool (level 1) (Morning) - He's in the locker room section.
Jinafire Long - Level 1 North Hall (Morning) - Just outside door to Swimming Pool
Skelita Calaveras - Level 1 North Hall (Morning) - Just outside door to Swimming Pool
Holt Hyde - Level 1 West Hall (Morning) - Outside door to Creepateria
Heath Burns - Level 1 West Hall (Morning) - Outside door to Creepateria
Cleo De Nile - Level 1 West Hall (Morning) - At the T-Junction
Deuce Gorgon - Level 1 West Hall (Morning) - At the T-Junction
Rochelle Goyle - Study Howl (level 1) (Morning) - On a computer
Lorna McNessie - Study Howl (level 1) (Morning) - On a computer
Scarah Screams - - Study Howl (level 1) (Morning) - Standing up near the computers
Slomo Mortavitch - Study Howl (level 1) (Morning) - On a computer
Tor-Bot (doesn't get added to contacts) - Bell Tower (Morning) - Keep ringing the bell with the switch at the top of the Bell Tower until he drops out from the bell. He'll give you 1000 coins, also.
Marisol Coxi - Behind the stage in the fashion club quest where you have to choose what type of dress, Apple Green or Midnight Blue
Jackson Jekyll - Study Howl (After School)
This is the entire list.