From S01E06 of the anime adaptation of the manga The Quintessential Quintuplets:
The male protagonist Fuutarou Uesugi is a smart but poor high school student who is tutoring these 5 (currently 3 in image below) quintuplets who are transfer students to Fuutarou's school and are in the same year as Fuutarou. They don't really respect Fuutarou as if Fuutarou were a tutor who were 1 decade older than them or anything. Each of the quints has their own (initial) way of addressing Fuutarou. The reason for the differences is a spoiler, but I believe of all of which would be how they address Fuutarou :
(in birth order)
- Uesugi-san (or Uesuuuggiiii-saaaaannn)
The eldest is Ichika Nakano who indeed calls Fuutarou by first name (Fuutarou-kun).
In the scene depicted below, Ichika jokingly addresses Fuutarou as 'Fuutarou-sensei' (Futaro-sensei / Fūtarō-sensei).
- (In the same scene, Fuutarou jokingly addresses them -kun eg Yotsuba-kun as if Fuutarou was their actual teacher / sensei as opposed to merely their tutor / kateikyoushi. cf 'And,some instructors who are elderly often use くん regardless of gender not to make a difference between boy and girl.')
Question: What are some situations where you can use 1st name with the honorific -sensei ?
I don't think I've ever seen in an anime, manga, VN or whatever, other than aforementioned, where 1st name is used with sensei. Even like outside fiction like say behind the scenes stuff it's still last name eg [Eng Sub] Konomi Kohara has a request for Akasaka-sensei - Kaguya-sama wa Kokurasetai for Aka Akasaka.
My own experience:
I never attended a Japanese school (or tutorial centre or had a Japanese tutor), but it varied for the schools I attended.
Pre-university, most schools were 'Mx / Mr / Mrs / Ms / Miss Last name' while some were 'Teacher 1st name'. This is really how we addressed them in our greetings to them at the start / end of classes and so this is how we referred to them among each other and to eg department secretaries. But, we addressed them as just 'sir'/'ma'am'/'madam'/'madame' or 'teacher'/''cher'.
- So anyway yeah, among the teachers that were 'Teacher 1st name', I guess those would be analogues of Japanese '1st name-sensei', if such a thing exists: Like are there actual schools in Japan that tell kids to say 'Ohayou gozaimasu, Fuutarou-sensei' instead of 'Ohayou gozaimasu, Uesugi-sensei'?
In university, we didn't greet our teachers / instructors / professors at the start and end of class anymore, so technically they were in the registration forms as 'Prof / Dr / Mx LAST NAME, 1st Name Middle Initial.' (eg Prof LORENZO, Jose Martin Q.), but we didn't necessarily have a uniform way to refer to each instructor.
- If they say their nicknames on syllabus day, then we might refer to them as Dr / Prof / Mx nickname (or 1st name) among each other or even to department secretaries. As for addressing them it could be 'sir'/'ma'am' with or without a name or still 'Dr / Prof / Mx nickname'. This part seems like another analogue of Japanese '1st name-sensei', if such a thing exists: Like are there actual universities in Japan that allow students to say to department secretaries 'Please call Jolo-sensei', where 'Jolo' is a nickname of LORENZO, Jose Martin Q. (if need be, then change this to your favourite Japanese name + nickname. Eg Ina-sensei for Ichika Nakano) ?
In some tutorial centres, the kateikyoushi / tutor might have a title either 'Teacher' or 'Mx'. And then the kateikyoushi may be addressed by 1st name (or nickname) or last name. But these were really just kateikyoushi's not sensei's. Maybe in Japanese tutorial centres (or other tutorial arrangements like in TQQ), I can imagine they can be about as informal as outside Japan esp if the tutors are young. Maybe here they'll indeed say like Fuutarou-sensei here? But, wait, would you use sensei in a tutorial case actually? Maybe there's a lower honorific? idk.
I did take a Japanese course in uni, but we just called the instructor 'sir'/'ma'am' and asked the department secretaries to call 'Mx Last name'. For fun, we just called the instructor sometimes Last name-sensei.
- Wait omg I just remembered after class ended, the instructor told us we can address h as either just 1st name or 1st name-sensei. Idk, so, what, is that legit? I'm not sure if that's really a Japanese thing or a combination of the Japanese honorific-sensei with the informality in my alma mater's country. Or maybe it's largely in part because the instructor is just a few years older than us. After some classes in uni, might some (former) students address their (former) instructors as 1st name-sensei?