Jargon is specialized wording that is not easily accessible by people who aren’t in the know. That includes words like fellowship, sanctification, and even salvation. Even normal words used in an unusual way, like brother, can count as jargon. My main concern when using these words is that it makes God feel inaccessible, bookish, and irrelevant. What's worse is that if your students pick up this stuff, walking into your youth group can feel like walking into some weird cult or foreign country to those on the outside.
While it is important for them to learn what many of these words mean, keep in mind that your audience has not spent nearly as many years in church as you have. If you use these specialized words, make sure you define them, but try to use easily understood synonyms when possible.
And yes, I see the irony: "jargon" is usually jargon.