Exactly which days of which month do Japanese high schools that use a 3-semester format (which can cause students to repeat a grade) typically hold midterms in the first semester? And how many days pass between taking the midterms and the posting of the grades?
In episode 177 of Sailor Moon, Ami (Mercury) and Taiki (Star Maker) tie for top grade when grades are posted in the hallway. Taiki mentions in this episode that the (fictional) comet they see is visible at the same time as the (real-life) Centarus constellation, which is visible in Tokyo from mid-March until mid-May, with its maximum visibility around the first week of April.
However, in the previous episode, 176, Nezu (Sailor Iron Mouse) is reading issue 7 of STAGE芸能 magazine, implying that it is mid- to late-June (since Japanese magazines come out a month, or half a month, before their publication date: 7 = July) and class 1-1 has a sho tesuto (quiz) in Math.
In the following episode, 178, it must be before June 1 because the students have not done koromogae yet (changing from long-sleeved school uniform to short-sleeved, which I read on the web takes place on June 1st every year). On the other hand, in this episode, Yaten (Star Healer) is taping a TV show, and I read online that Japan has four television seasons: winter (January–March), spring (April–June), summer (July–September), and autumn (October–December), and that each episode of Japanese dramas are usually shot only a few (two to three) weeks before they are actually aired. The minor character Noriko accuses Yaten of "itsumo" (always) heading straight home after a taping, so they're a number of episodes into the series. This would indicate that episode 178 is, at the earliest, in early June so that the episodes can air starting in July.
I imagine that episode 177 is correct regarding the time of year for grade posting, and that episodes 176 and 178 are the ones that makes a mistake about the timeline.
I found this info about the Japanese high school academic year online:
Typically, the first day of a new school year is around April 8th, 9th. Summer vacation usually starts just before July 20th and usually lasts til the last day of August. The students take their midterm exams at the end of May. Since the finals are given in July, that means the students only have a month and a half between midterms and final exams.
While midterm exams go on for two days, final exams last three days. For a week before both midterms and finals, school club meetings and other after-school activities are canceled. Students are also not allowed in the teacher's workroom or the copying room.
In high school, there are 5 periodical tests in a year (the midterm and the finals). There are some subjects which only require 1 test per term, some 2, others give no test at all, and some would base the student’s grade on classroom performance alone. Periodical exams are usually held within 3 to 5 days. There are some schools that give take-home exams or evaluation exams.