I went the route that I think ErictheRed is talking about myself, of getting a good general understanding of the opening and middlegame first and only then (when I was already at 1900) starting to take opening theory more seriously.
But that was a rather slow route, and I've seen a lot of juniors achieve excellent results and progress by adopting openings that are specifically chosen to be
much more difficult to play for the opponent than for themselves. Openings with easily explainable plans and/or tactics (I'm not talking about massive amounts of theory) that are either a headache to play against, the cost of mistakes is higher for the opposing side (the so-called Calthrop coefficient), or the typical opponent won't be prepared for it.
This line of reasoning leads to openings like the Benko Gambit, the Albin, the Accelerated Dragon and the Portuguese Gambit for Black, the Scotch Gambit, the English Opening and the Trompowsky for White (the last two because Black amateurs are hardly prepared for them).
Yes, this involves going into some quite specific position types, whether primarily positional or tactical, before the general chess understanding is strongly developed. But I'm no longer convinced that's a bad idea. This approach does bring results (because it targets the typical weak points of the opposition) and there's nothing like success to motivate further learning and progress. You might just start a self-reinforcing improvement spiral and gently direct the student towards more main line openings along the way.
And to finally return to the thread topic, my opinion of the French Advance for amateurs and juniors should now be obvious: Don't go there. Those positions are just what Black French players want, they play for typical plans like attacking d4, breaking with ...f6, exchanging off the bad bishop, sacing the exchange on f3 etc., and White's best moves with so much time invested in building a centre are much harder to find since they require prophylactic thinking. So 3.e5 will hurt results and therefore motivation.
The exception is the Milner-Barry Gambit, which might be a fine choice for a prospective attacking player since it's so easy for Black patzers to lose a miniature. In the modern 9.Nbd2 line it's not even clear that White is theoretically worse!