It's understandable that you're frustrated, but you've been training to dunk since you were 12? It'll pay off, be proud.
The only thing we can tell from these videos is:
- You look light/lean
- You are definitely strong relative to your bodyweight
- Your squat form is good
- Your squat is fairly fast, which is good, but it could be faster. For example, you'll want to keep slowly & safely increasing your squat strength so that your 315 x 1 moves much faster, and then eventually your 2xBW squat singles complete in approximately <= 2s. So you have some good speed, but it could be better - the easiest way to do that is to get stronger. For athletic performance training, a high relative bodyweight squat matters more when it can be completed fast & not "grinded out". Athletes can have "grind out maxes" and "fast maxes", just like they can have "competition maxes" and "training maxes". Ideally you want to be moving around 2xBW singles like a toy. Towards the "elite level", you would be moving 2.5xBW around like a toy.
- Be careful always training with people who goof off - such as those potentially in the first video. It's infectious and can actually hinder your training. I've seen this many times in basketball weight rooms / basketball team training. There's an enormous difference (usually) between football & basketball weight rooms. If i'm way off, sorry; but just something to look out for.
So as far as your most important strength exercise goes (squat), it seems like you are on the right track. I personally disagree that you need quarter squats and such right now, I imagine their are other explanations for your vert not being higher. Quarter squats are usually prescribed to advanced athletes who have "already matured". It's not usually prescribed to younger athletes. If you do experiment with quarter squats, make sure you definitely "kill your ego" a bit. It's very easy to add some serious load to partial ROM movements - and thus riskier (to the spine/knees etc).
Other things to make sure you don't neglect in the weightroom is calves (high rep 3-5x20, forefoot elevated, assistance), and upper body exercises for explosive arm swing (general upper body strength - weighted dips/pullups, and more specific exercises like plate swings & heavy med ball throws/tosses for shoulder power). The biggest thing we're missing though, is video of your jumps. You could be freakishly strong in the weight room, but your runup, plant, and arm swing might be too 'slow'. Do you jump single leg or double leg? I'm personally not big on training technique, but these would all be helpful to see/know.
When you say can't dunk, is that simply off of a dribble? Would a lob count? I imagine you want to land an off-the-dribble dunk.
When do you attempt your dunks? Before, after, or during basketball? In sessions all by themselves? All of the above?
Ideally you'd like to be attempting to jump maximally 2-3x/week, IMHO. If you do this already, but don't have an actual dedicated jump sessions (focusing entirely on maximal effort jumps and dunk attempts), i'd make sure to get one of those in per week. In a maximal effort jump session you will want to: warm up very good (dynamic warmup or basketball dribbling/layups), make sure you CNS is firing by doing several short sprints (baseline to 3-pt line or half court line, full recovery between each) & optionally timing them with a stopwatch so that you know you are fired up and ready to go, starting off with short approach (standing vert up to 2 step runup) max effort jumps, 3-step runup jumps, and optionally 4 step run-up jumps. Finally, once you've performed 2-3 max effort jumps at each level, you end the session with dunk (dribble or lob) attempts until 'considerable fatigue'. Sometimes you will find yourself fatigued & think the session is over, but really you might just need a bit longer recovery and to regroup mentally. You might find that you might have some really good jumps left in you. "Considerable fatigue" would basically be your legs feeling like they are about to give out, or several consecutive bad jumps. Usually you are "toast" when you attempt a jump and you feel your legs buckle etc. Hard to recover from that.
As for reactive exercises, I like the exercise someone else posted; multi response tuck jumps. Those are a good exercise. A slightly more difficult and perhaps more effective exercise, to be used in combination or in the next step in a progression, are multi response stiff legged pogo hops. I've found these to be extremely effective in getting more explosiveness out of the last range of motion of a jump; the triple extension phase. Stiff legged pogos really stress the calves/achilles, quads (vmo especially) and glutes. Those are usually done in the 5 x 10-20 range. If you decide to try this exercise out, do not go max effort for several sessions. You want to learn how to 'lock' your ankles/knees as you land and time your arm swing so it's an extremely spring-like movement. This makes much more sense when they are practiced submaximally for several weeks, before increasing intensity. I would stay away from more advanced plyometric exercises like depth jumps for now, unless they are from low boxes (~12-18"). There's a great bit of 'technique and timing' involved in these reactive exercises, always start light & relaxed. The first cue should always be 'get off the ground as fast as possible'. Eventually (as you become familiar with the exercise over several sessions), you want to focus on two simultaneous cues: get off the ground as fast as possible AND get as high as possible. Both of these cues together have been found to be more effective than focusing on one without the other.
Finally, if you are a single leg jumper (which I kind of doubt), you would also want to eventually learn how to altrnate leg bound, single leg bound, and perform heavy stepups/lunges with a barbell or dumbells. Learning to bound on your own is do-able but difficult, it's best to perhaps seek out your high school track & field coach for help with these exercises (and utilize a proper progression).
Finally finally, a good warmup (several rounds of ~1 minute) or "reactive work finisher" (post lifting session, ~10 minutes) is jump rope. It's good for an overall cardiovascular effect but also as a nice way to learn to better relax and reflexively spring. It's also good for arm speed.
In a nutshell, my advice is to keep improving squat safely & slowly, improve relative bodyweight squat speed, improve upper body strength (even if it's "just" dips and pullups), incorporate explosive shoulder power exercise, don't neglect calves, perform some reactive exercises after a warmup and before lifting (MR tuck jumps, MR pogos), incorporate jump/dunk only sessions - make sure your cns is fired up by utilizing sprints as a warmup (optionally timing them to make sure you are ready to go), don't get greedy (incorporate more advanced lifting techniques & plyo exercises later on down the road: after appropriate progressions & buildup duration).
I've typed alot so i'll leave it there for now, that's my 2 cents.