If you want to enter the Academy of Fine Arts, it is very important to get rid of the influence of Japanese cartoons when practicing sketching. If you don't plan to enter the Academy of Fine Arts, it's no problem to sketch in Japanese comics. In fact, Japanese manga is essentially a sketch.
Your teacher said "painting bad hands" is based on your confusion between realistic paintings and Japanese cartoons. There are too many Japanese cartoons, which will naturally replace the sense of brushwork in sketch.
In fact, real professional painters can draw various styles. I think if you practice more in both directions, you won't have bad hands. This is not accurate or appropriate. There is a smell of intimidation. The teacher said this just to prevent you from being influenced by Japanese cartoons in the college entrance examination art, but it's not that absolute.
The problem is your orientation and control. There's nothing wrong with drawing more. Any painting will help you understand the way they refine lines. If you want to avoid the influence of Japanese painting, the key is the treatment of face shape, not the generalization of body shape. Therefore, as long as you take good care of your face, there will be no "bad hands" phenomenon that teachers are worried about.