This covers both magical and weapons training, I hope, and is general enough to apply to either one.
Groups of people or mentors who train the heroes are really popular in fantasy (especially in the last few years, no small thanks to Harry Potter). They have their own unique problems, though, often caused by the author trying to write too much about it at once. Here’s some thoughts that have popped into my mind while reading about such training sessions, and which I hope will be helpful.
1) Limit the hero’s “natural talent.”
Someone can be “naturally talented” at sword-fighting, singing, magic, or whatever you want your hero to learn, but this should not be the equivalent of “astounds his teachers from the first day, already knows everything important, picks unimportant skills up effortlessly.” Even the most talented singers have to train. Even the most talented pianists have to pick at the keys on their first try. And sword-fighters are often shown sparring in fantasy, though somehow that fails when it comes to the hero who can fight well the first time he picks up a sword. Don’t make your hero into a genius at the one skill he happens to need to defeat the Dark Lord. (The Harry Potter equivalent would have been Rowling making Harry already somehow know all about magic and Defence Against the Dark Arts long before he went to Hogwarts, and the teachers just being there to add a little polish. She wisely did not do this). Struggles make your hero easier to emphasize with and bring him a little closer to the human from the superhuman.
Keep in mind that in a lot of disciplines, native talent doesn’t work on its own. It’s supported by theories, physical methods (in the case of things like sword-fighting or magical systems that use physical gestures), and historical facts. It won’t do the heroine much good to have the most powerful musical magic in the world if she doesn’t know that singing this note next to that note does nothing. She might be ahead of other students; she should not be ahead of her teachers, even if they have less “native talent” than she does.
2) Keep in mind that some skills are dependent on age.
Children learn languages better. Someone who starts learning to dance or fight at eight will be (and should be) far more comfortable with the dance or the blade than the hero who starts learning when he’s sixteen. An older teenager who’s growing fast may have to switch weapons as his balance changes.
Many, many fantasy protagonists are teenagers, and unless the author has already placed them in a situation where they’ve learned their skills from childhood, they’re going through a period in their lives where their rapidly changing bodies should affect what they can do. It doesn’t often seem to, though. They’re miracle, genius people, and don’t have to build up strength or adjust; it just somehow comes to them.
There are some disadvantages to having a teenage protagonist, and this should be one of them.
3) In an academy with different subjects and theories, don’t make your hero a genius at all of them.
This smacks of unrealistic plotting, again, and actually more so when you’ve given your hero one strong native talent. Say he likes making things float around in the air. It’s likely he’ll concentrate on that, practice it of his own free will, and become really good at it. That means he won’t spend as much time on, say, turning water into ice. If he doesn’t, he shouldn’t be as good at turning water into ice as he is at making things float around in the air.
One excuse often used to bypass this is that of “Well, I was (or knew) someone in school who never studied and still got good grades in all the subjects.” This doesn’t cover everything, particularly not in another world where the students are subject to much more physical stress than, say, a modern American high school student. It also doesn’t cover the complexity of some of the things the hero will be learning. Most of the people who breezed easily through high school without studying run smack into trouble in college and more complex subjects. (I know. I was one of them). Your hero might be able to walk through simple subjects, but not complicated ones.
4) Give your mentor a compelling reason to actually stay with his or her student.
As bratty as a lot of fantasy teenagers are, there doesn’t seem to be a compelling reason other than the mentor’s superhuman kindness and wisdom and compassion. They still act human around most other characters, though, which makes their excepting a teenager on whom a lot usually depends a bit of a puzzle. Why exactly is the mentor coddling the teenager’s outbursts instead of telling them they have to save the world, so grow up and stop whining? Or, if someone else is available to take the duties, why doesn’t the mentor leave?
The mentor-student relationship is, unfortunately, one of the most underdeveloped ones in fantasy. It’s assumed the mentor is the wise old wizard or bluff old soldier type, and the student will somehow grow up one day and see the errors of his or her ways (though often without ever apologizing to the mentor). Try to make it more than this, and make your mentor more than Gandalf. Gandalf is a wonderful character, but he’s not the only prototype for a mentor available.
5) For the love of all the gods, don’t turn every other student around your hero into a bully, or someone jealous of the hero’s skills.
I’ve seen this happen constantly, and it bothers me. Yes, probably most of us remember a bully picking on us. But probably most of us are beyond the stage now, can realize that high school is not the world, and that the bullies had their reasons; they aren’t automatically less intelligent or more shallow people because of the role they played in high school. This goes back to not making your heroine’s perceptions the same as the world that surrounds her. She can think that someone is only bullying her because he’s jealous of her talent at magic, but she should not be 100% right. If the other students are made into shallow caricatures because of the limitations of the heroine’s viewpoint, fine. They should not really be shallow caricatures, however.
There are many, many more interesting relationships that can be explored in a school setting than just those of “best friend” and “jealous, shrieking, taunting bully who I’ll show up someday.” Try to give your heroine some unattractive characteristics, and the friendships and rivalries some deeper ones.
6) Come up with a realistic amount of time for the training to take, and keep mentioning it.
One of the best depictions of this I’ve ever seen in fantasy is in Tad Williams’s Memory, Sorrow, & Thorn Trilogy. Simon, the hero, doesn’t start learning to fight with a sword until he’s 15, and it takes months. Williams emphasizes all the tiredness, bruises, wounds, and calluses he gets from the training, and he keeps mentioning that it continues. This is a problem a lot of authors have; the training lapses in the business of the Quest, and doesn’t ever resume enough to explain how the hero is somehow a shining swordfighter at the end of it.
If you’re writing about an academy, then do have scenes where the heroine is in a class and learning something new, rather than just having all the scenes with the classes take place elsewhere. It’s an interesting way to provide variation, and can give the incredible things the heroine turns out to be able to do at the end of the book some kind of basis.
It’s amazing how many people are writing about schools now because of Harry Potter, and doing everything that Rowling avoids so neatly.