What was the previous stage you did, before you were supposed to talk to Judy? I can tell you what you need to do next, because I've been compiling a guide while I play. At no point in Celia's route do you need to talk to the Principal.
If it's early in the route, you need to grab the letter from the table in Judy's office (by the door), then use the letter with the pen from the locker with the heart, and drop the letter in the locker in the breakroom.
If it is late in the route, you need to go to Celia's house at night and go to the living room to spy on Celia to find out what she likes, then return to Judy.