The fastest way to recruit students from other houses in Fire Emblem: Three Houses
When you play Fire Emblem: Three Houses, one of the most important aspects of the game is the separation of school houses at the monastery. Whichever house you pick locks you into a certain roster of units and characters to fight and interact with throughout the game.
There is a very important mechanic in the game, however, that allows you to recruit characters from the other houses for use in your own army. In doing so, not only do you get new units to use for the rest of the game (for the most part), but you also have the option to diversify your ranks, or perhaps fill spots that are lost if any of your units are killed off. If you're playing on Casual, there's obviously less cause for concern there, but that can be useful on Classic.
Recruiting, as you've probably figured out by now, can be a tedious process. Each student you wish to acquire, with some exceptions in certain knights and faculty members, require you to meet a minimum level in certain skills. If the student you want to recruit is interested in lances and speed, for instance, you need to have high skill power in lances and speed to recruit them. This can take some serious time, especially for would-be recruits that ask for high skills that you have no reason to train in. If you're a Brigand, for instance, you probably don't have much need for high faith. There is an easier way, though.
Recruitment guide: Tea parties are the key
You want to know how to recruit students outside of your house easier? Easy, invite them over for some tea.
Well, that's only the partial truth. Tea parties just happen to be one reliable way to boost up your support rank with other units, and you simply want to boost your support rank to make this happen. More specifically, ranking up your support level with any given character, lowers their requirement and makes it much easier to recruit them. As far as we can tell, getting a character to C rank lowers the requirement, and a support of B rank seems to allow you to recruit them no matter what. We have a success rate of 100 percent with nine characters so far. It appears there are a few characters who can't be recruited at all, but most of them can, and this seems to work.
So the question then becomes: How can I boost my support rank with characters outside of my house? This is a much easier question to answer, and one you may already know the answer to. Simply get gifts you think they'd enjoy, return any lost items they have, invite them to tea parties, have successful tea parties, cook food with them, invite them to eat food they enjoy, and so on and so forth. The list of possibilities for growing your support rank is vast, so if you find the unit you want to recruit, it shouldn't be too difficult of a task.