Back squats are tough. Add a quarter rep to each one, and they get a lot more difficult. But, as you already know, if it doesn't challenge you, it doesn't change you. To perform 1-1/4 squats, get under the bar and lower yourself all the way down. When you're in the hole, press up one quarter of the way, lower all the way back down, and then press the weight all the way up until you're in a standing position.
These reps should be heavy, but they shouldn't be so heavy you can't complete the sets and reps. If you're not sure how much to put on the bar, start with something you know you can hit for about 10 reps. If you can get 6 of the 1-1/4 reps easily, add some pounds. When you start to fail on that sixth rep, you've hit the right weight.