You and your wife should be a team, and if she expects you to be "fully invested" in being a parent to her son she should support you.  Putting pressure on you to help and then not supporting you, sucks big time.  I'd tell her if she's not willing to present a united front to SS, then you won't be helping with him any further. 

This ^^^^

Your wife doesn't get to cherry pick. She either wants you invested and wants your help, which means supporting you when you need to put your foot down, or she can just deal with him and his education by herself.

I don't disagree with step parents being fully invested (if they so choose), especially when the kid was so young at the start of the relationship, but that means you get to be fully invested in discipline, also.

Sounds like she's fully invested in having you do the heavy lifting but not really be able to parent. I'd let her know that either you get full parenting privileges and she backs you up, or you stop helping with his school work.

IMO, your wife is the biggest problem  You need to have a heart-to-heart discussion with her and ask for her full support.  She needs to understand (1) how lucky she is to have a spouse who cares about her son’s education and is willing to help (your love and devotion to your SS is not typical from what I've seen); and (2) what a difficult position she’s put you in by not supporting you.

With regard to you stepson, I’d recommend that you adjust your expectations.  ADHD is a really hard on the kid and their parents.  Try not to get frustrated when things don't go according to your expectatoins, as there are some learning challenges that are beyond his (and your) control.  There are plenty of 2e (twice exceptional) kids who are both gifted and have ADHD and go on to have very bright futures, but the path is not without challenges.  

Your DW is the main problem.  She cant cherry pick how SHE want you to parent.  Either you fully parent, in all parts of SS life and she supports you, or you disengage from the circuse,    She only shan't you to do as told by her now.  This is not the way to live,  With your DW playing her games 

I'm curious as to why Dad isn't as "invested". Is that something that is his choice, or did your DW decide that she didn't like him as a dad and picked you as a replacement, relegating Dad to every other weekend and a few weeks around the holidays?

No adult should have responsibility without authority over a child. You can't be responsible for their welfare without having the authority to ensure that the child can actually be taken care of. That includes discipline and conseweucnes for behavior. As others have said, your wife can't cherry-pick that she'll have you do the hard parenting work but then not give you any ability to reinforce the behavior you're trying to parent.

The solution? You stop being so invested. Help with homework if you want and have time, but Mom needs to be responsible for Junior's grades, missing assignments, checking his work, talking to teachers, etc. That's HER job. It's also DAD'S job. If she has pushed Dad away, then you need to not be available so that she has to rely on Dad. It's both his responsibility and right to be invested, and if she is taking that away to give to you, you need to drop it so Dad can pick it up. And you should encourage her to involve Dad, or at least not alienate him from the process.

If Dad is absent by his own choice (and not influenced by your wife's behavior - my DH backed off on a lot of things when BM and her clan responded with unfounded CPS calls, requests for money to pay for EVERYTHING in excess of child support and not related to the kids, lies about DH to anyone that would listen to her, and attempted alienation of the kids from DH through lies), then it's STILL your wife's job to handle the parenting, up to 100% on her own. That's her consequence for breeding with someone who was not parent material. She doesn't get to choose a new partner and then force them to take on her responsibilities.

Look at it this way: if you are your wife were a sports team, you two would be players in different positions. You work together, but you each have a different role and responsibility. She's not the coach who dictates what the team does. Having a kid doesn't make her the "leader" ; it just means her role and set of responsibilities is different.

Additionally, her son is 100% her responsibility in your home. You becoming her husband didn't diminish her responsibility to him or cause that to spread across the two of you. She STILL had 100% responsibility, and you are ADDED onto that. You're not there to pick up her slack. There shouldn't BE slack that you need to pick up when it comes to SS because she should have had that handled before she started dating.

Ultimately, you can't force her to give you authority. Many have tried, and many have failed. The only thing you can control is YOU. So if you're frustrated and she's not willing to hand over some authority, you have the right to let it go and no longer help. That's the "leverage" we have as stepparents - we CAN say "no, not my responsibility, not my problem". If your wife is foolish enough to leave you over something like that, then it's her loss because very few people would be willing to put up with that long-term.