The school is first alluded to in "Partner's Complaint", when Daria snarks about him going to it. Jane says that Tom's parents "make him go there"; presumably this is how Tom described it to her. Daria was unimpressed by this excuse.
Fielding has an honour roll, but (unlike Lawndale High) doesn't have trimesters. ("Is It Fall Yet?")
In Kara Wild's Driven Wild Universe, Fielding is where Alfred Phelps, Quinn's math teacher, once taught. In "Charge of the Math Brigade," Jane tells Daria: "[Phelps] may be a stuffed shirt, but he's as big a stuffed shirt to the smart ones as to the idiots. He treats us all as equally stupid. Tom thought so, too." Later, in "Into the Fire," Daria learns some troubling details about Phelps's past from a Fielding administrator, Marshall Winsett. Fielding winds up playing a significant role in the final Driven Wild Universe fanfic, "Tomorrow Never Knows."
Roentgen's extensive and popular fanfic "The Hallowed Halls of Fielding", starting in 2010, has Daria and Quinn go there instead of Lawndale High (where Daria makes friends with Elsie instead of Jane). It would develop a massive amount of lore - history, geography, and culture - and dozens of pupils & teachers. He allowed other fans to use elements from his work. Ever since then, if Fielding shows up in a fanfic, it is almost certainly some variation of Roentgen's take.