There are Guardian Academies in each of the 7 mega cities and are where (while not required) all new MG are taught everything they need to know about how to be an MG.
The existence of the Guardian Academy is common public knowledge, in part as high school students take a class on MG culture. But mostly because of the annual summer festival that each Academies holds. These festivals are city-wide events with days of games, events, and tournaments, that are heavily publicized and broadcast on TV.
“Classes there are taught by either experienced Guardians, Familiars, or subject matter experts. There, you’ll learn things like proper fighting technique, the lore behind magic, the common types of Anathema and how to recognize them, marketing yourself as a Guardian, working in teams of Guardians, and a variety of other things.” Ever ch.29
“It is highly recommended that all Magical Girls go through at least one year of the academy, preferably all three. The survival and growth rate of a Guardian increases dramatically for each year you attend. Additionally, there are many special opportunities that can only be attained by attending.” Selene ch.29
On Guardian Academy class scheduling:
“There are a few options, one is to go to only the academy and receive private tutoring on the normal school subjects. This is something I wouldn’t recommend. Immersing yourself with only Magical Girls can be incredibly insulating and skew your perspective negatively. It also robs you of any chance of normalcy, something we as Guardians need to balance out the stressful, combat filled lives we live. No, the normal route is to go to a normal school on Monday, Tuesday, and Wednesday and then attend the academy on Thursday, Friday, Saturday.” Ever ch.29
Of note this schedule is the same as SO students.