Frieren and Eisen trained Fern and Stark, but were they actually that good at teaching and learning?
Frieren: Beyond Journey's End (Sousou no Frieren) has several main characters, and two of them are apprentices. Fern is Frieren's human apprentice, and Stark is the apprentice of Eisen, a dwarf who went on that 10-year adventure with Frieren to defeat the Demon King.
We will try not to spoil much, but some details are very important to understand if Frieren and Eisen are good teachers and if Fern and Stark are good apprentices.
The Trial by Fire
Frieren and Eisen's teaching become especially relevant in Episode 6, but even before that, we can see how Frieren teaches Fern. She covers the basics with her, gets her to read important books, and they also train. But an important aspect of being a hero is to be able to face different monsters, and in Episode 6, Fern reveals how Frieren taught her to do that. It was basically not intervening despite Fern being terrified and running away from the monster. One does not doubt that Frieren was nearby and ready to intervene, but she waited until Fern stopped running away and destroyed the monster, which Frieren knew Fern could do.
That was a training in courage, and while it can be considered very questionable, it did its work. Frieren: Beyond Journey's End has an interesting relationship with fear. It does note that normally, heroes would scoff at the idea of being afraid.
But in the end, the series suggests that fear might be a good thing because it lets you live on. As such, being courageous or, at least, confident in oneself does not mean that you do not get scared sometimes and flee the battlefield.
When Frieren trains Stark to get that confidence, the situation is very similar. Fern and Frieren watch Stark completely demolish the dragon, which lived near the village where Stark was celebrated for driving that very dragon away. Stark does not even realize that he killed the thing and berates Frieren for not helping until she tells him that he did it. The monster is dead.
She adds quite a bit of praise to that announcement, which is a good teaching practice. But is it good to leave apprentices to deal with monsters alone?
That is a very difficult question to answer because in real life, there are no monsters. So when students get individual assignments, the assignments are not likely to leave you half-dead. One can argue that the method proves to be effective and the teacher knows what a student can and cannot do.
One can also say that it is irresponsible and involves needlessly endangering students. But within the laws of the fantasy word, it makes sense, and Frieren seems like an effective teacher. After all, Fern is a great, powerful magician due to Frieren's instructions.
But what about Eisen? Is he a good teacher?
He seems to be in many ways. For one, Stark might leave him before he combatted his fears, but he does appear extremely well-trained and also disciplined; he keeps training throughout the years that he is away from his teacher. Unfortunately, and Eisen will be the first to say that, they parted on a wrong note. Eisen had hit Stark, and while Stark has his own explanation for why, Eisen admits that it was because he got afraid of his student. It is a great shame that Eisen carries.
To be fair, Eisen took Stark in because he saw himself in the boy, especially in their struggles with fear. And Stark has so many positive memories of Eisen traveling with him and sharing sweets with him, that it cannot possibly be argued that Eisen was a bad teacher. He definitely made a mistake when he hit Stark and then let him leave without making up for that, but Stark's memories are proof of Eisen's kindness.
Indeed, Eisen does not have to worry because Stark is a good apprentice. He plans to help Frieren on the new ten-year adventure, but he wants to hurry so that he can visit Eisen before the old man surrenders to death. He remembers every good thing Eisen has ever told him; he did grow up to be a marvelous fighter, and he loves Eisen, so he hopes they meet again.
Fern is a good apprentice as well. Aside from the fact that she is quite accomplished in magic and heroism, she is also a good influence on her teacher, Frieren. She helps Frieren with small tasks, encourages her to be less lazy and controls the elf's greedy attempts to take everything she can from the dragon's nest.
You could argue that the characters we are shown in Frieren: Beyond Journey's End are imperfect but have good intentions. As such, Frieren and Eisen are both good teachers, even if they make mistakes sometimes. And there is no doubt that Fern and Stark are good apprentices because aside from becoming good at their craft, they also love and respect the ones who got them there.