Front squats may be a better alternative to back squats
Doing barbell front squats with my client Serena. I find the front squats to be much easier to teach and for clients to perform then barbell back squats. So many people have tight hip flexors and underdeveloped or weak core muscles that it makes it difficult to perform barbell back squats in good form. Because of this they tend to have a lot of forward lean or flexion at the waist. This can cause the pelvis to tilt forward or anteriorly a lot and put stress on the lower back. This ends up causing poor form and for the exercise to be performed incorrectly.
Front squats are good because they help eliminate the forward lean and keep the pelvis in a more neutral position. If you have too much forward lean at the waist you will drop the bar in front of you. This is usually much safer then dropping the bar in the back squat and having the weight roll of your back. Front squats also help people keep the core more tight and the torso more upright. Both of those will help perform the movement in the proper form.
Like any exercise when the form starts to slip you want to end the set. This was Serena’s last set and her form started slipping on the last rep so we ended the set.
I should note that I usually don't have clients go above about 10 reps on front squats. This is because when the reps start to get higher the shoulders, arms and upper back tend to end up fatiguing before the legs and then someone never gets to thouroughly work the legs and other working muscles.
If you are having trouble performing barbell back squats correctly or any other squatting movement I recommend trying to do barbell front squats. You may find it easier to perform.