Is it bad to hold a stool until a convenient time?

Hard, bulky stool. Children who avoid a stool for whatever reason can later forget to empty. This may result in a hard or bulky stool that is painful to pass. To avoid pain, younger children may try to avoid having stools & older kids may just want to avoid going at school. Over time, this can weaken the emptying function & cause a recurrent problem with slow transit/poor emptying & require specific program to fix.
No and Yes. No, on rare occasions and often necessary. Ever once in a while, this is no big deal. However, repeated on an ongoing basis, this can lead to significant bowel dysfunction, constipation and encopresis.