If it's something they've noticed, then it can be a sign of other things, and Social Services may be interested.
Not saying this is applicable for you, but a front garden full of rubbish can be an outwardly visible sign of neglect, much like 'kids that are always dirty' might also get reported - may be from neglect, or may be that a neighbour sees the kids when they're out playing in their assigned getting muddy clothes, and on the walk home after they've been round the woods! Can feel very 'someone being a busy body' for something reported, but if there was a case of abuse or neglect, and school have said 'not had clothes that fit for a while' and a neighbour has said 'outside the house is a tip' and another neighbour has said 'The kids are out in the back garden really late' it can add up to a worrying picture, which SS would be right to investigate, and then determine whether any support is needed! This is also probably why school are SS first point of contact for most investigations, as they see kids day in day out, know what parents are like, and are able to build up more of a full picture.
In case you received any further contact, have you got evidence where you've reported missing collections? But I imagine that, SS will contact the school, who will comment that they've noticed no signs of neglect, will let them know if the kids come in clean clothes, generally tidy, homework done, fed properly etc, and SS will then write to you and say 'we've contacted the school, who've let us know they've got no concerns about neglect, and we're closing the investigation'.