When assigning a group of pupils to a detention or wanting to move a group to a different detention, other detention slots will show red if one or more pupils are in that detention.

If this is escalating a detention to the next afterschool detention, if you be useful to add some pupils to the next one available, then assign another detention for the pupils already in the next one for a different reason.

See screen shot, where I am simply unable to add the whole group as one or more are already in the afterschool dentation.