So I have been wait-listed at two of my colleges so far (a REALLY huge surprise), and I was wondering if I would be able to retake the ACT or SAT to improve my score (I GUARANTEE that my score or super-score will go up) while on the wait-list. Any response would be greatly appreciated.
THANKS! 
Bump
Most likely a score from a spring 2017 test date would not be accepted, but check with the school.
^This. I don’t think it would be accepted, but check with the school. You’re allowed to inform them of any major updates, such as awards you’ve won–maybe they’ll let you give them a better test score.
I wouldn’t count on it, though.
How can you guarantee a higher score?
Yeah buddy, check in with the school. Solid question and if you can, binge on practice material and understand.
I think, personally, it’s a waste of time. You are on a waitlist along with hundreds of others. Suddently cramming to get a higher test score, now that decisions are out, is not impressive. If anything, my first thought would be “why didn’t they do this before, and submit that higher test score in the first place?”
Many students on waitlist would have been originally in the accpeted pile, and regrettably moved to the waitlist pile because usually students have to be culled. The kids called from waitlist first will be those students. Maybe you are in that pile, or maybe not, but I doubt a suddenly higher test score is going to win you admission over a kid that maybe already has a high test score, but wasn’t initially admitted for whatever reason.
You are far better off sending a follow up email explaining that it’s your first choice and you will attend if admitted. Explain why you are a good fit for the school, and update with any achievements or honors. I don’t think a new higher test score is the kind of achievement that will impress them.
I saw on one school’s site that past January/February test would be possibly too late to be considered for admissions (not sure if it’s the same for waitlisted people). So, I think March and beyond is most definetly too late. Plus, It probably wouldn’t affect much since an improvement is expected for taking the same standardized test in multiple sittings.
I have a 36 ACT and was waitlisted so far at UChicago, Caltech, and Harvey Mudd. At a certain point colleges won’t care if your scores are a little higher than the next guy’s. Sorry bud. 
^^^What @stlgirl99 said. If you’re waitlisted, it is not because your score was too low. You’re much better off sending something new, like an award or just an enthusiastic letter promising to attend if you’re accepted.