The rule exists because this is the MARRIAGE thread. Not the "Pick up random weak minded strange" thread. This is a thread dedicated to people who have chosen to live a monogamous long term relationship with someone and sometimes the fallout of that decision. This isn't the "Should I Eiffel tower my current GF before dumping her?" thread. That is elsewhere.
There doesn't need to be a thread for that. It's always yes, if you're about to dump her anyway, eiffel tower her and while she's got the other dude's cock in her mouth, give her the dolphin maneuver.
Now on to wormie's dilema. Just going to spitball some ideas. I understand how a mind can obsess over things. Especially things that it can't have. The problem is that simply denying yourself something usually makes you obsess over it even more. So you need to break the obsession cycle.
1. Are you a christian or religious in any way? I'm going to guess no, and it's too bad because if you were you could get help from someone in your church. But you could still use the same concept they'd probably use and get yourself an accountability buddy (or if you're familiar with AA, a sponsor). You've kind of done that by coming here, but it needs to be someone you know IRL who actually cares a little about you. Even though we're mostly giving good advice, most of us want to see naked pics of your 2 girls more than anything else.
2. I agree with whoever it was that said you should tell the girl you want to fuck her and let her shoot you down. Just be sure you're strong enough to push her away if she goes "OK". If she keeps flirting with you after that, then you know she's pure poison and you can do what you need to do. Whether that be dropping classes or staking her in the heart or something in-between.
3. Figure out some way to make her unattractive. How sharp are her knees? Did you just see her eat a booger? Maybe you can friendzone her if you think of her as a sister.
4. What would happen if you had her over to hang out with you and the wife? Invite her to the next social event. That way at least she wouldn't be a secret.
Oh and 5. You're a dumbass for thinking this semester of school is more important than your marriage. Seriously, stop fucking deluding yourself. I don't care if you're a time traveler from the future and you must have your degree finished in time to stop skynet from becoming sentient. Your marriage is more important than ONE semester of school.