I want my students only to hit ribs at specific angles, knees, clavicles and jaw how do you practice this in sparing?

You can't!.

There would be no one to practice against. You can't even do this in a realistic manner with a red man suit on.

Only those with a sports mentality attempt to make such feeble argument. They are too blind to admit that there are differences in training for sports and training for self defense.

As you've figured out it's very difficult.

Hitting the ribs is not a good idea and I'd advise against it.

Going "full force" isn't a problem IF your students are advanced, have great control and technique. For beginners and intermediate student it's just not possible without having someone get hurt. For more advanced students (senior brown belts and up) it can be developed but takes time.

Prearranged sequences are good except that if done poorly develop the mentality of "he does this, I do this, he'll do that, and I'll do this other thing". And this can be a disaster in the real world when someone does something else and the person doesn't expect it.

So while not idea, you have to mix things up. You have to have kata, exercises, drills, prearranged and finally free sparing. You learn to use the tools to develop different skills and go from there.

Oh and in "prearranged sequences" you can add in screw ups. In other words, someone doesn't do the combination as prescribed, so the partner has to be on guard and ready for everything.

The key is to figure out what is going to help your students and don't do things that will hurt them.

BTW one last thought. If you're having them hit each other in the jaws, knees and such be prepared for someone to get hurt. It's not a good idea to be doing that and in time it will happen.

Unless you have someone who is willing to get hurt and get his limbs broken you can never practice full out. That's the way it is. I don't really see the problem. If you are sparring and doing your thing. Then let's say that something happens and you find yourself in a situation where you have to defend yourself. Is it so hard then to hit a little harder?

You would have to use a modified sparring method, such as slowing down the movements like Taiji Chuan or TFT.

good answers overall