He might be a bit unstable but he’s also about the smartest and most capable dedicated teacher in his entire school. Never get on his bad side though.

Though he’s technically a student, he’s also a mentor and guide to juniors and does a better job than most professional teachers. He’s clearly the best and most steadily competent fighter/ magic user there is. His guidance is quite effective in getting the best out of anyone he mentors or even simply acts as a role model for but he can also be lazy and a closet super perve despite his holy position which makes for some fun and hilarious antics.

He’s essentially perfect but he’s surprisingly humble and fails to realize his own worth. He’s about the closest manga /light novel character to Professor Snape of Hogwarts fame.. Both are: Intimidating; dark and brooding; have similar backgrounds and prejudices thrown against them; are at home in the shadows; are excellent yet surprisingly caring teachers; have ties to similar entities; have a similar espionage like role; are exceedingly smart and talented; and are favored or loved by children(though for Snape it may be more the actor than the character he played). There are many parallels in both stories. This novel/manga is from a Snape like protagonist/character perspective but there’s also a Harry Potter like character and his friends aka Aiden, who is also somewhat of a unique powered genius and also wears glasses. His skills as a teacher is beyond exceptional and surprises everyone but Rudgar himself. He’s effectively too humble for his own good.

Like Koro, he almost always seems to know what students are upto, thinking, or hiding.  He's extraordinarily protective of not just his students but children in general and when either are threatened is a rare tine where essentially goes berserk and goes well beyond normal power, abil, and skills to punish the offenders.  His attitude may appear aloof and indifferent but it often results in especially privileged students learning equality, humility respect, and even confidence and individuality they never had the chance to learn.  Despite his initial efforts to discourage attending his classes, students basically rush in crowds to join and pack his classroom.

He’s a natural talent with lots of practical hands on experience that translates well to the lessons he gives.

Heine is a prodigy level genius but still technically younger than most of his students.  He's skilled in adaptability needed to gain insight into the minds of his students and understand what motivates and drives them to eagerly self improve and heed his teachings.

He's a genius but also a playful otaku who can't get away from childish interests.  At the same time, his childish nature gives him unprecedented insight into the minds of children that are his students with which combined with his high tier intellect makes him a formidable teacher to try to deceive or misbehave against.

He’s easily one of the best at what he does in the entire series and capable of challenging if not beating even the toughest enemies alone. He’s smart and caring about his students and knows his subject matter very well.

She’s a bit vindictive and even perhaps sadistic in her methods but she teaches two of her would be friends how to overcome their faults and fears.

Koro is the ultimate OP teacher who is superb/superior in nearly everything he does.  It's curious that despite his dark past he's turned into about the most morally upright being around.  No matter what he teaches or frankly how, his lessons are duly imparted upon his students, he succeeds, well eventually at least.  His methods while at times may seem unconventional totally engages all under his charge.  His ability to predict, understand, and address any problems the students have is virtually beyond belief.  He's all around super smart and super powerful.  Like Shinigami, he's capable of shapeshifting and chooses a rather child friendly goofball form to put them somewhat at ease compared to what he's capable of or what form he's used before.

She may look like a loli but she’s not. She just lost much of her power, but she’s also actually a very good teacher and quite skilled though a bit clumsy.  It's ironic that her efforts to overpower the heroes ends up benefiting them in a similar way Gokudo does in his show (Though not as a teacher but an anti hero type who's evil backfired on him and benefits for the good.  Tanya on this list is a similar type of character.).  Still Mao looks like a loli but is much older than her appearance as her deep knowledge and experience help attest.  In this since she has similar characteristics to Heine, but her style is more of the clumsy goofy airheaded genius who knows her specific topics well but lacks the practical application experience that anyone who isn't an otaku would have.

I’d really only say she’s sort of an honorable mention because though not technically a teacher, her ability to provide guidance to others especially key characters is quite successful for the betterment of all who follow said guidance.

While Qi may not exactly be fun well not in the traditional sense, he’s undoubtedly one of the best teachers as he essentially makes the lead characters of the anime kingdom what they are, though the main character unfortunately seems to turn stupid in season two and somehow forgets a a large portion of his training and experience.

Her unconventional if not a bit unhinged nearly childish behavior totally throws her students off guard.  In her school, people with such abilities as she teaches form teams, but she can do it on her own thus proving she's earned her teaching chops.  She's sort of the do'er type teacher who can do it all and expects her students to follow by the example she leads.  Her antics are often both cute and humorous if not a bit confusing or annoying at times but she grows on you.

She technically becomes a teacher at one point and she’s pretty smart and capable. Though she’s not the strongest in the show her ability to learn and adapt makes her the most dangerous if you’re on her bad side. Her insight gives her the best chance to understand what she is and how to balance life among the humans. It sometimes puts her at odds with both her friends and potential enemies. Her flexible nature and ability to convey things she comprehends is well above the norm.

While he’s ostensibly the principal, he often teaches lessons to students though it is mainly of a more philosophical nature. Like Korosensei, he displays a goofy child friendly form of himself but also has a scary form that can challenge most any enemy. He’s exceptionally smart and acts as a guardian guide that watches over students to ensure safety and that they are learning.

She often acts like a strict adult librarian who reminds one of a strict aunt or mother who isn’t afraid to administer corporal punishment as readily as lessons about manners and behavior.

Though some may not agree, Tanya acted the part of a senior student to help educate at her school and later trains/teaches the most powerful and elite squad of mage knights anywhere. Her methods maybe a bit abrasive and she maybe selfish but her antics are more than quite effective at turning her troops into hardcore bad asses like her. She’s shown a level of compassion and caring despite her hate of being X and her claims of not caring. The anime makes her seem more hardcore than the reading sources.

She’s passionate about both the subjects she teaches and her students plus she’s got a cool hidden secret identity.

Technically though he’s not officially a teacher, his function as a squad leader and pseudo diplomat often has him teaching both his troops interesting lessons and the other worlders he befriends. There are even some scenes where he bonds with other otaku types and teaches them or those with him a few anime/manga type facts.