I'm a little confused as to why everyone assumes that they DEFEAT the injustice league..
Couldn't they lose? Couldn't they look like they stand a chance for...about 5-10 minutes? Than JL comes in and saves them?
That would be awkward, since they're the stars of "YOUNG JUSTICE" and we basically had such a thing happen in "HOME FRONT" a mere two episodes prior. Which doesn't even get into their loss to Clayface at the start of "DOWNTIME" which still views very abruptly.
I suppose it could end in a stalemate of sorts and the Injustice League escape or something. I can't imagine the show just having Joker show up once and be arrested, and Vandal Savage may or may not be on "The Light". Still, some of them would likely go down, like Poison Ivy or Count Vertigo. The rest of the IJ, from that screenshot, are Black Adam, Ultra-Humanite, and Wotan.
If the stars of your TV series lose too often, they seem incompetent. That's usually Writing 101. Of course, there are exceptions. The titular "INSPECTOR GADGET" often contributed so little to the plots of his episodes besides gags that some people have edited episodes that exclude him and focus on Penny, and the plots usually remain intact. Some have also claimed that when you think about it, Indiana Jones didn't accomplish a whole lot in "RAIDERS OF THE LOST ARK". For the entire film, he sought to keep the Ark out of Nazi hands, yet he fails, which works out fine because it melted all their heads anyway. About all he did was ensure Marian survived and that a few extra Nazi got dead than usual. That didn't make it a worse film, of course. The casts of AVENGERS ACADEMY and to a degree the old NEW MUTANTS rarely had any outright wins; the young heroes usually survived more often than outright won, but it managed to work.
The risk of over-using the JL in this show is that while they're a key part of the show's universe and several members of the team are mentor figures to the cast, if they take over too much screen-time or can ALWAYS arrive in a crisis, then it ruins drama.
I mean it's a weird thing. I actually am anticipating this episode heavily, because it looks to be a good hero vs. villain throw down where everyone gets a moment and whatnot. In fact I'm probably looking forward to this episode more than I have the others lately. I think it'll rock regardless of whatever nitpicks I poke at. But then I still keep looking at the premise and wondering how the episode will handle it. The League (Batman especially) seem foolish or reckless sending in the Junior Squad against a team of their worst villains knowingly, and said team of worst villains will look stupid if they lose to the Junior Squad.
Again, for an example, there is NOTHING that Kid-Flash should be able to do that the Flash can't do better at this stage, so a villain who is prepared for Flash should have little trouble competing with his sidekick. Aside for cackling and wordplay, Robin doesn't offer much against Joker (or Poison Ivy) that he (and she) hasn't seen done better by Batman. What can Artemis show Count Vertigo that he hasn't seen from Green Arrow already? To the Injustice League, YJ should be like fighting the low rent knock offs of their enemies. And I don't mean to come down hard on the YJ cast, because I do like the characters and I do think the concept works. I prefer this approach than what "TEEN TITANS" did. Just the odd advantage of "TT" was that it deliberately made itself so simplistic that flaws became difficult to criticize because they were intentional parts of the universe - akin to complaining that "BATMAN: BRAVE AND THE BOLD" is too silly or campy, when it is DELIBERATELY so. "YOUNG JUSTICE" has taken a more serious and steady approach, which I prefer. It just sets the bar higher, is all.
The dilemma with being the sidekick to a mentor hero is that at least until said sidekick matures and becomes more defined, they really are just smaller versions of that hero. And that's fine when they're fighting oddball threats or single enemies, but against a team which in theory is used to fighting a squad of veteran Justice League members (especially the core 7, in all probability), that becomes a major handicap. Which means either YJ have to come up with some strategy to outwit the IJ, which makes the JL look stupid for not thinking of it (such as with AMAZO), or IJ have to job a little. There's always the element of surprise, and villains tending to underestimate teenagers, but that should only go so far. I'm not saying it's impossible for the episode's writers/producers to overcome these nitpicks - I was satisfied with how "HOME FRONT" was handled - I'm just addressing them.
It's just the nature of the beast. Sure, YJ may have Captain Marvel or whatever JL mentor of the week is with them, but that merely keeps Black Adam busy; he alone could destroy most teams of characters.
A future episode has Zatanna as a "new friend". Since Weisman shows rarely introduce major characters without build-up, maybe they meet her sooner, such as in this episode. Or some other additional guest character besides the mentor who can mix things up. I suppose Miss Martian could use her shape-shifting powers to pretend to be someone else for a while and catch the IJ off guard, but that assumes the group never assume Martian Manhunter is with whatever squad of Leaguers who challenge them. The ol' "switch opponents" trope with teams vs. teams should be employed at least once. Part of me is betting on seeing Superboy toss Joker or something.
Still, I suppose a threat like the Injustice League, who seem insurmountable for our lead heroes, is the sort of threat that usually works on these sorts of shows. If you can easily predict how the baddie will lose, than that's bad for superhero tales.
I expect some wisecrack about fighting two talking evil gorillas in a row from someone (Mallah last week, Ultra-Humanite here). I also wonder if Wotan, one of the most powerful evil wizards DC has, will bring up that awkward "Kid-Flash doesn't believe in magic" subplot. Yes, I've had people tell me "DENIAL" was more about Wally not seeing what is obvious rather than him not believing in magic - but the episode itself didn't make that clear and instead focused on him not believing in magic. I still find that an awkward non-romantic subplot idea for him. Say it gets resolved; Wally now believes in magic. Does it accomplish anything for him? No. Meanwhile, everyone else tends to have better subplots - Aqualad with his leadership issues and "father", Artemis with her own family issues, Megan with her own "is she a White Martian" past and so on.
Tidbit: Joker and Poison Ivy are the only DC villains who were actually in the Injustice League in the comics that're in it in this show, and neither were founders. The founding team had Catwoman and Penguin representing the Batman universe. Several other former comic book IJ members, though, have appeared on the show before.
After all, the show came THIS CLOSE to depicting the JL as incompetent as it took 14 of them 4 hours to defeat AMAZO, yet about five kids (with Artemis in the shadows shooting one arrow) managed to do the same in, what, 25% of the time? Of course, there are caveats - the JL went in blind about what AMAZO could do initially, and YJ capitalized on AMAZO trying to protect its creator - but, again, THIS CLOSE. Maybe that's why it was called "SCHOOLED". 
